Mind Blowing Questions With a Checked Answer on Every Card

174 questions whose true answers sound wrong.

Mind Blowing Questions With a Checked Answer on Every Card

How many people must be in a room before two probably share a birthday?

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Just 23 With 23 people there are 253 different pairs, and each pair is a chance of a match.

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Scale of the universe 64

  • Are there more stars in the Milky Way or trees on Earth? Answer: Trees, by most estimates. A 2015 count put the world's trees at about 3 trillion. Estimates for the galaxy run from 100 to 400 billion stars.
  • Besides Earth, where in the solar system does rain fall into lakes and seas? Answer: On Titan, a moon of Saturn. The liquid is methane and ethane, not water.
  • Does the whole Sun spin at one speed? Answer: No. Its middle spins faster than its poles. One turn takes about 25 Earth days at the equator and about 36 at the poles. A ball of gas can do that.
  • Driving at 60 mph without a stop, how long would it take to reach the Moon? Answer: About 166 days. The Moon is 238,855 miles away on average. Divide by 60 and you get about 3,981 hours.
  • Driving straight up at 60 mph, how long until you reach space? Answer: About an hour. A common line for the edge of space is 100 km (62 miles) up, so the drive takes 62 minutes.
  • How bright is high noon on Neptune? Answer: Like dim twilight on Earth. Sunlight here is roughly 900 times as bright as it is out there.
  • How dense is the center of the Sun compared with gold? Answer: About 8 times as dense. NASA gives the core about 150 grams per cubic centimeter. It is still not solid.
  • How far is one light-year in miles? Answer: About 5.88 trillion miles (9.46 trillion km). It is the distance light travels through empty space in one year of 365.25 days. It measures distance, not time.
  • How far through its life is the Sun? Answer: A little less than halfway. It formed about 4.6 billion years ago and has about 5 billion years to go before it becomes a white dwarf.
  • How fast do winds blow in Saturn's upper atmosphere? Answer: About 1,600 feet (500 m) per second. The strongest hurricane winds on Earth top out at about 360 feet (110 m) per second.
  • How heavy is the black hole in the middle of the Milky Way? Answer: About 4 million solar masses. It is called Sagittarius A*, said A-star.
  • How hot is a bolt of lightning compared with the surface of the Sun? Answer: About five times hotter. Lightning can heat the air it passes through to 50,000 °F.
  • How long does energy made in the Sun's core take to work its way outward? Answer: About 170,000 years. That is the trip from the core to the top of the convection zone. The last leg to Earth takes about eight minutes.
  • How long does the Sun take to go once around the Milky Way? Answer: About 230 million years. It carries the whole solar system along at about 450,000 mph (720,000 km/h).
  • How long does winter darkness last at a pole of Uranus? Answer: About 21 years. One Uranus year is about 84 Earth years, and each pole spends about a quarter of it facing away from the Sun.
  • How long has the giant red storm on Jupiter been raging? Answer: For hundreds of years. NASA describes the Great Red Spot as a storm bigger than Earth.
  • How long is the biggest canyon on Mars? Answer: About 2,400 miles (3,870 km). Valles Marineris is long enough to stretch from California to New York, more than 10 times the length of the Grand Canyon.
  • How many Earths could you line up between Earth and the Moon? Answer: About 30. The Moon is an average of 238,855 miles (384,400 km) away.
  • How many Earths could you pack inside a hollow Jupiter? Answer: About 1,000. NASA's picture: if Earth were a grape, Jupiter would be a basketball.
  • How many Earths would fit inside the Sun? Answer: About 1.3 million. By mass it is a different story: it takes about 330,000 Earths to match the Sun.
  • How many full trips around the Sun has Pluto made since it was discovered? Answer: None yet. It was discovered in 1930 and one orbit takes 248 years, so the first full lap ends in 2178.
  • How many planets in our solar system have rings? Answer: Four. Jupiter, Saturn, Uranus and Neptune all have them. Jupiter's were found by Voyager 1 in 1979.
  • How many sunrises does the crew of the space station see each day? Answer: About 16. The station laps the Earth about every 93 minutes.
  • How many times could light circle the Earth in one second? Answer: About seven and a half. Light covers about 300,000 km each second and the equator is about 40,075 km around.
  • How much of all the mass in the solar system is in the Sun? Answer: About 99.8 percent. Every planet, moon, asteroid and comet put together shares what is left.
  • How much of the universe is ordinary matter, the stuff of stars, planets and people? Answer: About 5 percent. About 27 percent is dark matter and about 68 percent is dark energy. Nobody yet knows what either is.
  • How old is the light from the nearest star beyond the Sun? Answer: About 4.24 years. Proxima Centauri is 4.24 light-years away, so you see it as it was more than four years ago.
  • How old is the light you see when you look at the Andromeda galaxy? Answer: About 2.5 million years. It set out long before there were people. Our species is about 300,000 years old.
  • How old is the sunlight landing on your face right now? Answer: About eight minutes. Light needs that long to cross the 93 million miles (150 million km) from the Sun.
  • How old is the universe? Answer: About 13.8 billion years. The oldest light we can observe set out about 380,000 years after the big bang.
  • How tall is the solar system's largest volcano? Answer: Nearly 22 km (about 14 miles). Olympus Mons on Mars. A laser altimeter on a Mars orbiter measured it at 21.287 km (69,840 ft), roughly 2.5 times the height of Mount Everest above sea level.
  • How wide is Pluto compared with the United States? Answer: About half as wide. Pluto is only about 1,400 miles across.
  • How wide is the Milky Way's disk? Answer: More than 100,000 light-years. Earth sits along one of the galaxy's spiral arms, about halfway out from the center.
  • If all of Earth's water were rolled into one ball, how wide would it be? Answer: About 860 miles (1,385 km). That is a bit more than the distance from Salt Lake City, Utah, to Topeka, Kansas.
  • If Earth were the size of a nickel, how big would the Moon be? Answer: About as big as a coffee bean. The Moon's radius is about 1,080 miles (1,740 km), which makes it less than a third of the width of Earth.
  • If the night sky has stars in every direction, why is it dark? Answer: The universe has not been around forever. Light from the most distant stars has not had time to arrive. The puzzle is known as Olbers' paradox, after an 1823 account.
  • If you dropped Mount Everest into the deepest part of the ocean, would the top show? Answer: No. It would be over a mile underwater. The Challenger Deep is about 10,935 meters down. Everest is 8,848 meters tall, which leaves 2,087 meters of water on top.
  • If you rolled every asteroid into one ball, how would it compare with our Moon? Answer: It would have less mass than the Moon. Vesta, the largest asteroid, is only about 329 miles (530 km) across.
  • Is any moon bigger than a planet? Answer: Yes. Ganymede is bigger than Mercury. Ganymede orbits Jupiter and is the largest moon in the solar system.
  • Is Pluto always farther from the Sun than Neptune? Answer: No. Pluto's oval orbit brings it inside Neptune's for about 20 years out of every 248. It last happened from 1979 to 1999.
  • Is the Moon staying put, drifting toward us, or drifting away? Answer: Drifting away, about an inch a year. At an inch a year the gap grows by one mile about every 63,000 years, because a mile is 63,360 inches.
  • Is there a side of the Moon that is always dark? Answer: No. One side always faces away from Earth, but it gets as much sunlight as the side you see. Far side is the better name.
  • Mercury is the planet closest to the Sun. Is it the hottest? Answer: No. Venus is hotter. Venus has a thick atmosphere that traps heat. Mercury has almost none, so its nights drop to -290 °F (-180 °C).
  • Neptune was found in 1846. When did it finish its first lap of the Sun since then? Answer: In 2011. One trip around the Sun takes Neptune about 165 Earth years.
  • On which planet can the Sun rise, set, and rise again in one morning? Answer: Mercury. From parts of the surface, Mercury's oval orbit and slow spin make the Sun appear to rise briefly, set, and rise again.
  • On which planet does one day last two of its years? Answer: Mercury. Sunrise to sunrise takes 176 Earth days. A Mercury year is only 88.
  • On which planet does the Sun rise in the west? Answer: Venus. Venus spins the opposite way from Earth, so sunrise is in the west and sunset is in the east.
  • One twin rockets away near light speed and comes back. Which twin is older? Answer: The one who stayed home. The twin paradox of special relativity. The trips are not mirror images: only the traveler changed direction.
  • Saturn's rings reach 175,000 miles out. How thick are the main rings? Answer: About 30 feet (10 meters). The rings are billions of chunks of ice and rock, from dust-sized grains to pieces as big as a house.
  • Sunlight reaches Earth in 8 minutes. How long does it take to reach Saturn? Answer: About 80 minutes. It gets to Mars in about 13 minutes and to Jupiter in about 43.
  • What causes summer and winter: Earth's distance from the Sun, or its tilt? Answer: Its tilt. Earth's axis leans 23.4 degrees, so each half of the planet gets more direct sunlight for part of the year.
  • What did the astronomer who discovered Uranus think he had found? Answer: A comet or a star. William Herschel spotted it in 1781. It was the first planet found with a telescope.
  • What in the middle of the Earth is about as hot as the surface of the Sun? Answer: The inner core. NASA puts it as high as 9,800 °F (5,400 °C). The Sun's surface is about 10,000 °F (5,500 °C).
  • What is the coldest that anything can possibly get? Answer: -459.67 °F (-273.15 °C). It is called absolute zero, and it is zero on the Kelvin scale.
  • What shape is the jet stream at Saturn's north pole? Answer: A hexagon. It is a six-sided pattern in the clouds.
  • What would the air pressure on the surface of Venus feel like? Answer: Like being over 3,000 feet underwater. The pressure at the surface is about 93 times the pressure at sea level on Earth.
  • Where might you find ice on the planet closest to the Sun? Answer: In craters at Mercury's poles. Some crater floors there are in permanent shadow, cold enough to keep water ice.
  • Which is hotter: the Sun's surface or the thin outer atmosphere above it? Answer: The outer atmosphere, by a long way. The surface is about 10,000 °F (5,500 °C). The corona above it reaches up to 3.5 million °F (2 million °C).
  • Which planet has a day that lasts longer than its year? Answer: Venus. One spin takes 243 Earth days. One trip around the Sun takes 225.
  • Which planet has a surface hot enough to melt lead? Answer: Venus. It is about 872 °F (467 °C) down there, which makes Venus the hottest planet.
  • Which planet has the shortest day, and how short is it? Answer: Jupiter, at about 10 hours. The biggest planet is also the fastest spinner. NASA gives one turn as 9.9 hours.
  • Which planet is so light for its size that it would float in water? Answer: Saturn. It is the only planet with an average density below that of water. You would need a very big bathtub.
  • Which planet rolls around the Sun on its side? Answer: Uranus. It is tilted 97.77 degrees, possibly by a collision with an Earth-sized object long ago.
  • Who came up with the name Pluto? Answer: An 11-year-old girl. Venetia Burney of Oxford, England, suggested it to her grandfather in 1930, and he passed it on to the observatory.

Deep time and living things 40

  • A peanut is not a nut. So what is it? Answer: A legume. Peanuts are in the pea and bean family, and the pods ripen underground.
  • About 70 percent of the world's fresh water is in one place. Where? Answer: Frozen in Antarctica. If it all melted, the seas would rise by almost 60 meters (200 feet).
  • All the continents were once joined. When did that supercontinent start to split? Answer: About 225 to 200 million years ago. It is called Pangaea, which means all lands in Greek.
  • Are any dinosaurs still alive? Answer: Yes. Birds. Birds evolved from small meat-eating theropods, the same group T. rex belonged to. Bird-like fossils go back over 150 million years.
  • Could people once walk from Britain to mainland Europe? Answer: Yes. The land now under the southern North Sea is called Doggerland. Rising seas broke it up by about 7,000 years ago.
  • Did Cleopatra live closer in time to the Great Pyramid going up or to the Moon landing? Answer: To the Moon landing. The pyramid went up around 2600 BC. She was born in 69 BC, about 2,500 years later and about 2,040 years before 1969.
  • Did T. rex live closer in time to Stegosaurus or to you? Answer: To you. Stegosaurus lived 155 to 145 million years ago and T. rex 69 to 66 million years ago. The gap between them is the bigger one.
  • For how much of our species' history has anything been written down? Answer: Under 2 percent of it. Writing first appeared in Mesopotamia around 3400 to 3100 BC, about 5,400 years ago. Our species is about 300,000 years old.
  • For how much of our species' time on Earth have people farmed? Answer: About the last 12,000 of 300,000 years. That is about 4 percent. For the rest, everyone found food by gathering, hunting and fishing.
  • Has a compass needle always pointed north? Answer: No. Earth's magnetic field has flipped many times. The last full reversal was about 780,000 years ago. They come at random, and each takes hundreds to thousands of years.
  • Has the Mediterranean Sea ever dried up? Answer: Yes, almost completely. It happened in cycles between 5.96 and 5.33 million years ago, and ended when the Atlantic flooded back in.
  • How do we know Earth's age if its oldest rocks were destroyed long ago? Answer: From meteorites. Meteorites formed with the solar system and were never recycled. Their lead isotopes give 4.54 billion years.
  • How fast is the floor of the Atlantic Ocean spreading apart? Answer: About 2.5 cm (1 inch) a year. That is the average along the Mid-Atlantic Ridge. It adds up to 25 km in a million years.
  • How long can a Greenland shark live? Answer: At least 250 years. One large female was estimated to be between 272 and 512 years old.
  • How many hearts beat inside one octopus? Answer: Three. One sends blood around the body and two pump it through the gills.
  • How much lower was the sea at the peak of the last ice age? Answer: About 125 meters (410 feet). So much water was locked up in ice sheets that coastlines sat far out from where they are now.
  • How much of your DNA differs from a chimpanzee's? Answer: About 1.2 percent. That counts single-letter changes in shared genes. Any two people differ by about 0.1 percent.
  • If Earth's history were squeezed into one day, when would our species appear? Answer: About 6 seconds before midnight. Earth is about 4.54 billion years old and Homo sapiens about 300,000. That is 300,000 / 4.54 billion of 86,400 seconds.
  • To a botanist, which one is a berry: a banana or a strawberry? Answer: The banana. A botanical berry grows from a single ovary of one flower. Bananas, grapes and tomatoes qualify. Strawberries and raspberries do not.
  • Were woolly mammoths still alive when the Great Pyramid was built? Answer: Yes. A small population lasted on Wrangel Island until about 4,000 years ago. The pyramid dates to around 2600 BC, some 4,600 years ago.
  • What did the Sahara look like 7,000 years ago? Answer: Much of it was green, with lakes and rivers. During the African humid period, from about 14,500 to roughly 5,500 years ago, grass, trees and lakes covered most of North Africa.
  • What in Utah looks like a forest of 47,000 trees but is one living thing? Answer: Pando, a quaking aspen. Every trunk is a genetically identical stem of one root system that spans 106 acres.
  • What is the largest animal that has ever lived? Answer: The blue whale. It is bigger than any dinosaur found so far, and it lives on krill.
  • What is too small to see yet makes up to 20 percent of the oxygen in the biosphere? Answer: A sea bacterium, Prochlorococcus. It is the smallest photosynthetic organism on Earth, and it out-produces all the tropical rainforests on land combined.
  • What kind of rock is at the very top of Mount Everest? Answer: Limestone from an ancient seabed. Samples from near the summit contain bits of trilobites and other sea creatures.
  • What single living thing in Oregon covers about 3.5 square miles? Answer: A honey fungus. It lives in the Malheur National Forest, spreads from tree to tree through black root-like cords, and is put at some 8,000 years old.
  • What was the day job of the man who proposed that continents drift? Answer: He was a meteorologist. Alfred Wegener set out the idea in 1912, at age 32. Most geologists did not accept it for decades.
  • When did the first dinosaurs appear? Answer: Around 245 million years ago. That is in the Triassic Period. The non-bird dinosaurs died out about 66 million years ago.
  • Where does about half of the oxygen made on Earth come from? Answer: The ocean. Most of it is made by drifting plankton too small to see.
  • Where was India 225 million years ago? Answer: It was an island off the coast of Australia. It drifted north across an ocean and ran into Asia. The slow crash pushed up the Himalayas.
  • Which blue-blooded beach animal is over 200 million years older than the dinosaurs? Answer: The horseshoe crab. Its copper-based blood clots around bacterial toxins, so it is used to test that vaccines and medical devices are clean.
  • Which came first: sharks or trees? Answer: Sharks. The oldest shark-like scales are about 450 million years old. The earliest trees came around 400 million years ago.
  • Which continent is a desert buried under ice? Answer: Antarctica. It gets the equivalent of about 150 mm (6 in) of water a year, and the ice on top averages 1.9 km (1.2 mi) thick.
  • Which fish was known only from fossils until one turned up in a net in 1938? Answer: The coelacanth. It was thought to have died out about 66 million years ago. A museum curator spotted one in a fisherman's catch off South Africa.
  • Which is older: teaching at Oxford or the Aztec capital Tenochtitlan? Answer: Teaching at Oxford. There is evidence of teaching at Oxford as early as 1096. Tenochtitlan was founded in 1325.
  • Which jellyfish can turn back into its younger self? Answer: Turritopsis dohrnii. After reaching adulthood it can revert all the way to the polyp stage, which is why it is nicknamed the immortal jellyfish.
  • Which North American frog can survive its blood freezing? Answer: The wood frog. It winters under leaf litter, tolerates ice forming in its blood and tissues, and thaws out in spring.
  • Which salamander can regrow a lost leg? Answer: The axolotl. It can also regenerate its gills and parts of its eyes and brain.
  • Which tiny animal has survived a trip in open space? Answer: The tardigrade, or water bear. In 2007 dried-out tardigrades rode outside the FOTON-M3 mission, exposed to vacuum, and survived.
  • Who has more bones in their body: you or a newborn baby? Answer: The baby. A newborn has around 270 bones. Some fuse as the child grows, which leaves around 206.

Numbers and chance 40

  • 100 kg of potatoes are 99% water. They dry out to 98% water. What do they weigh now? Answer: 50 kg. The 1 kg of solid potato has to become 2 percent of the total, so the total must be 50 kg.
  • A bat and a ball cost $1.10. The bat costs $1 more than the ball. What does the ball cost? Answer: 5 cents. Most people blurt out 10 cents. Then the bat would be $1.10 and the pair $1.20.
  • A coin game pays $2, doubling for every tail before the first head. What is a fair price? Answer: On paper, any price at all. The average payout is infinite, yet hardly anyone would pay much to play. Nicolaus Bernoulli posed the St. Petersburg paradox in 1713.
  • A fair coin lands heads 10 times in a row. What is the chance of heads next time? Answer: Still 1 in 2. The coin has no memory. Expecting tails to be due is called the gambler's fallacy.
  • A hotel with infinitely many rooms is full. Can it take one more guest? Answer: Yes. Ask every guest to move up one room and room 1 is free. David Hilbert told the story in a 1924 to 1925 lecture.
  • A lily patch doubles daily and covers the lake on day 48. When was the lake half covered? Answer: On day 47. The quick answer of day 24 is the one most people give.
  • A million seconds is about 11 and a half days. How long is a billion seconds? Answer: Nearly 32 years. A trillion seconds is about 31,700 years.
  • A rope circles the equator. How much more rope lifts it 1 meter high all the way round? Answer: About 6.3 meters (21 feet). The extra is 2 times pi times the height, whatever the size of the ball. A tennis ball needs the same 6.3 meters.
  • An alarm is 99% accurate and real break-ins are 1 in 10,000 nights. It rings. Is it real? Answer: Probably not. About 1 chance in 100. Out of 10,000 nights, expect about 100 false rings and 1 true one. Ignoring that is called the base rate fallacy.
  • Are some infinities bigger than others? Answer: Yes. Georg Cantor published a proof in 1891: the decimals between 0 and 1 cannot be paired off with the counting numbers.
  • Are there more possible chess games than atoms in the observable universe? Answer: Yes, by a huge margin. Claude Shannon's 1950 estimate was 1 followed by 120 zeros games. The atoms come to about 1 followed by 80 zeros.
  • Are there more whole numbers than perfect squares? Answer: No. They pair off exactly. Match 1 with 1, 2 with 4, 3 with 9, and nothing is left over on either side. Galileo puzzled over it in Two New Sciences.
  • Are there two people in London with exactly an equal count of hairs on their heads? Answer: Certainly. London has far more people than any head has hairs, so some head counts must repeat. It is called the pigeonhole principle.
  • Boxes hold 2 gold, 2 silver, and 1 of each. You draw gold. Chance the other coin is gold? Answer: 2 in 3. There are three gold coins you could be holding, and two of them sit in the all-gold box. Joseph Bertrand set it in 1889.
  • Can a group prefer A to B, B to C and C to A, all by majority vote? Answer: Yes. The Marquis de Condorcet described it in the late 1700s. Three voters who rank ABC, BCA and CAB are enough to make the loop.
  • Can a shape hold a cupful of paint yet have a surface too big to ever paint? Answer: Yes, in mathematics. Gabriel's horn, also called Torricelli's trumpet, has a finite volume and an infinite surface area.
  • Can a treatment look better in every single group and still look worse overall? Answer: Yes. It is called Simpson's paradox, after a 1951 paper. It shows up when the groups are very different sizes.
  • Can building a new road make everybody's trip slower? Answer: Yes. Braess's paradox, named for Dietrich Braess in 1968. Each driver picks the fastest route for themselves and the shortcut clogs.
  • Can moving one item from one group to another raise the average of both groups? Answer: Yes. Move a value that is below average where it is and above average where it lands. Statisticians call it the Will Rogers phenomenon.
  • Can you comb a hairy ball flat with no tuft or parting anywhere? Answer: No. The hairy ball theorem. Treat wind as the hair and it says that somewhere on Earth there is always a spot with a cyclone.
  • Do your friends have more friends than you do, on average? Answer: Probably yes. The friendship paradox, first noted by sociologist Scott Feld in 1991. Popular people show up on more friend lists, so they skew the average.
  • Given endless time, would a monkey hitting keys at random type a whole play? Answer: In theory, almost surely. The infinite monkey theorem. In practice the odds are so small that the age of the universe is nowhere near long enough.
  • Has a well-shuffled deck of cards ever come out in the same order twice? Answer: Almost certainly never. 52 cards can be ordered in about 8 followed by 67 zeros ways, far more than the number of shuffles ever made.
  • How many colors are enough for any flat map so that no two touching regions match? Answer: Four. It took until 1976 to prove. Kenneth Appel and Wolfgang Haken needed a computer to check the cases.
  • How many decimal places of pi does NASA use to fly between planets? Answer: 15. With those digits, the distance around a circle 125 billion miles long comes out wrong by about half an inch.
  • How many people must be in a room before two probably share a birthday? Answer: Just 23. With 23 people there are 253 different pairs, and each pair is a chance of a match.
  • How many riffle shuffles does it take to mix a deck of cards properly? Answer: Seven. That is the usual advice from a mathematical model of how people really riffle cards.
  • If you could fold a sheet of paper in half 42 times, how thick would it be? Answer: Thick enough to reach the Moon. A 0.1 mm sheet doubled 42 times is about 440,000 km. The Moon is 384,400 km away. In real life you run out of paper fast.
  • In real-world figures like river lengths, how often is the first digit a 1? Answer: About 30 percent of the time. Benford's law. A 9 leads less than 5 percent of the time. Accountants use it to spot made-up numbers.
  • Is 0.999 repeating forever smaller than 1? Answer: No. It is exactly 1. One third is 0.333 repeating. Multiply both by three and you get 0.999 repeating on one side and 1 on the other.
  • Is every even number bigger than 2 the sum of two prime numbers? Answer: Nobody knows. Christian Goldbach asked in a letter to Euler in 1742. No exception has been found and no proof has either.
  • On paper, can one solid ball be cut up and put back together as two balls the same size? Answer: Yes. The Banach-Tarski paradox of 1924. The pieces are infinite scatterings of points with no definable volume, so no knife can make them.
  • One grain of rice on the first chess square, doubled each square. How many on square 64? Answer: Over 9 quintillion. That is 2 to the power 63. The whole board would hold 18,446,744,073,709,551,615 grains.
  • Pick a number. Halve it if even; if odd, triple it and add 1. Repeat. Where do you end up? Answer: At 1, every time anyone has tried. Nobody has proved that it always happens. Lothar Collatz posed the puzzle in 1937.
  • Seven bridges joined four parts of old Königsberg. Could one walk cross each exactly once? Answer: No. Leonhard Euler proved it in 1736. Each piece of land touched an odd number of bridges, and such a walk allows at most two.
  • Three doors, one prize. You pick. The host knows where it is, opens an empty one. Switch? Answer: Yes. Switching wins 2 times in 3. The Monty Hall problem, first posed by Steve Selvin in 1975. The host knows where the prize is and always opens an empty door, so your first pick stays right 1 time in 3.
  • What is the length of the coast of Britain? Answer: It depends on the length of your ruler. A shorter ruler follows more nooks, and the total keeps growing. Lewis Fry Richardson studied it and Benoit Mandelbrot made it famous.
  • Which is bigger: a googol or the count of atoms in the observable universe? Answer: A googol. A googol is 1 followed by 100 zeros. A common estimate for the atoms is about 1 followed by 80 zeros.
  • Who named the googol? Answer: A nine-year-old boy. Milton Sirotta, nephew of the mathematician Edward Kasner, came up with the word in 1920.
  • Would you take $1 million today, or a penny that doubles every day for 30 days? Answer: Take the penny. On day 30 it is worth 2 to the power 29 cents, which is $5,368,709.12.

Logic knots 30

  • A barber shaves everyone in town who does not shave themselves. Who shaves the barber? Answer: No such barber can exist. The story is tied to Bertrand Russell's paradox of about 1901, which asks the same thing about the set of all sets that do not contain themselves.
  • A card says 'the back is true'. The back says 'the front is false'. Which side is right? Answer: Neither can be settled. Philip Jourdain's card paradox. It is the liar loop stretched over two sentences.
  • A lamp is switched after 1 minute, then 1/2, then 1/4 and so on. Is it on at 2 minutes? Answer: The rules do not say. Thomson's lamp, 1954. Paul Benacerraf's reply: the story fixes every moment before two minutes and is silent about two minutes itself.
  • A man from Crete says that all Cretans always lie. Is that a paradox? Answer: Not quite. It can simply be false. If just one Cretan once told the truth, his claim is plain false. The line is credited to Epimenides of Crete.
  • A student owes his tutor a fee once he wins a case. The tutor sues him for it. Who wins? Answer: There is no agreed answer. The paradox of the court, told of Protagoras and his pupil Euathlus. Each man has an argument that he wins whatever the verdict.
  • A teacher promises a surprise quiz next week. The class proves it can't happen. Can it? Answer: Yes. It comes on Wednesday and surprises them. The surprise test paradox. Philosophers still argue over exactly which step of the students' reasoning fails.
  • Achilles must first reach where the tortoise was, again and again. Can he ever pass it? Answer: Yes. Zeno of Elea's puzzle. The catch-up steps shrink so quickly that infinitely many of them fit into a finite time.
  • Add 10 numbered balls to a jar, take out the lowest, repeat forever. How many are left? Answer: None, on the usual reckoning. Ball 1 leaves at step 1, ball 2 at step 2, and so on, so every ball has a leaving time. John Littlewood described it in 1953.
  • An author trusts every line of her book yet is sure it holds a mistake. Is that rational? Answer: Most philosophers say yes. The preface paradox, from D. C. Makinson in 1965. Being sure of each claim is not the same as being sure of all of them together.
  • An endless list of sentences each says 'every sentence after me is false'. Which are true? Answer: No choice is consistent. Stephen Yablo built it in 1985 to show that a paradox does not need a sentence that talks about itself.
  • At any one instant a flying arrow is in one place, not moving. So when does it move? Answer: Across instants, never inside one. Zeno's arrow. The modern reply: speed is about where the arrow is at nearby times, so a single instant cannot show it.
  • Cards show A, D, 4 and 7. Rule: a vowel has an even number on the back. Which two test it? Answer: A and 7. A and 4 are the tempting picks, but nothing on the back of the 4 can break the rule. In Peter Wason's 1966 study fewer than 10 percent got it right.
  • Does spotting a green apple help to prove that all ravens are black? Answer: In strict logic, a tiny bit. All ravens are black means the same as all non-black things are non-ravens, and a green apple fits. Carl Hempel raised it in the 1940s.
  • Does the list of all lists that do not list themselves include itself? Answer: It cannot, and it cannot be left off. This is Bertrand Russell's paradox in everyday dress. Modern set theory avoids it by not allowing such a list to be formed.
  • Each ticket in a million-ticket draw will almost surely lose. So will every ticket lose? Answer: No. One of them has to win. The lottery paradox, raised by Henry Kyburg in 1961. A million safe beliefs add up to one false one.
  • 'Grue' means green if first seen before 2100, blue if after. Are emeralds green or grue? Answer: Every emerald seen so far fits both. Nelson Goodman's new riddle of induction, from his book Fact, Fiction, and Forecast. Why do we trust green and not grue?
  • 'I went to the pictures last Tuesday, but I don't believe that I did.' Can that be true? Answer: Yes, but nobody can sincerely say it. Moore's paradox, after G. E. Moore. The sentence is fine about someone else. Said about yourself, it undercuts itself.
  • 'If this sentence is true, then the Moon is made of cheese.' What goes wrong? Answer: It seems to prove anything you like. Curry's paradox, traced to Haskell Curry in 1942. Assume the sentence is true and the cheese follows, which is exactly what it said.
  • Is the sentence 'This sentence is false' true or false? Answer: Neither answer holds up. If it is true then it is false, and if it is false then it is true. The liar paradox is traced to Eubulides, 4th century BC.
  • Is there a smallest boring number? Answer: No. Being the smallest boring number would make it interesting. It is known as the interesting number paradox, and it is mostly a joke.
  • One falling grain of millet makes no sound. How can a whole bushel make a thud? Answer: One grain does make a sound, just too faint to hear. Another of Zeno's puzzles, reported by ancient writers.
  • One grain is not a heap, and one more grain never makes one. So how do heaps exist? Answer: The word heap has no sharp edge. The sorites paradox, credited to Eubulides in the 4th century BC. The usual reply: vague words have no exact cut-off point.
  • One guard always lies, one never does. One question must find the safe door. What is it? Answer: Which door would the other guard call safe? Take the other one. Either guard ends up pointing at the wrong door. Raymond Smullyan named these knights and knaves puzzles in 1978.
  • 'Short' is a short word: it describes itself. Does 'non-self-describing' describe itself? Answer: Either answer contradicts itself. The Grelling-Nelson paradox, set out in 1908 by Kurt Grelling and Leonard Nelson.
  • Three friends pay $30, get $5 back, keep $1 each, tip $2. 27 + 2 = 29. Where is the $1? Answer: Nowhere. The sum is a trick. The $27 already includes the $2 tip. The honest sum is 25 for the bill, 2 for the tip and 3 handed back.
  • To cross a room you must go halfway, then half of what is left, forever. Do you arrive? Answer: Yes. Zeno's dichotomy. The pieces 1/2 + 1/4 + 1/8 and so on add up to exactly 1.
  • To use a rule of logic, do you first need a rule that says you may use it? Answer: No, or you would never finish. Lewis Carroll's 1895 dialogue What the Tortoise Said to Achilles. Each new rule would need another, so reasoning has to start somewhere.
  • What is the smallest number that cannot be described in under twelve words? Answer: That phrase seems to describe it, in eleven. Berry's paradox. Bertrand Russell printed it and credited G. G. Berry, a junior librarian at Oxford.
  • You know your father but not the masked man, who is your father. A contradiction? Answer: No. The Masked Man, also credited to Eubulides of Miletus. Knowing a person is not knowing every way he can be described.
  • You still have whatever you have not lost. You never lost horns. So do you have horns? Answer: No. The Horns paradox, credited to Eubulides of Miletus, a Greek philosopher of the 4th century BC. The first line only works for things you had to begin with.

How it works

These mind-blowing questions each hide a checked answer, such as Which came first: sharks or trees? The table guesses, then one press shows the answer and a note that says why. The 174 cards cover the universe, deep time, numbers and logic.

Read the full guide

Guess first, then show the answer

Press Next question and read the card out. Get a guess from everyone, then press Show answer. The Kind row picks one of four kinds, and the level row runs from Easy, which hurts a little, to Hard, which hurts a lot.

Four kinds of question

Scale of the universe has 64 cards, such as How many Earths would fit inside the Sun? Deep time and living things has 40, like Are any dinosaurs still alive? Numbers and chance has 40, from shared birthdays to a penny that doubles for 30 days. Logic knots has 30 old puzzles, each with its standard way out and, where one is known, the name of whoever first posed it.

  • On a how-many card, ask everyone for a number before the reveal. The closest guess keeps the card.
  • After a logic knot, ask who still disagrees with the answer. That argument is the game.
  • Pick a kind, then Print on the end card: each slip carries its answer, so a quizmaster can run the round from paper.

Questions and answers

What are some questions that will make your brain hurt?

Try a logic knot: Is the sentence 'This sentence is false' true or false? One grain is not a heap, and one more grain never makes one. So how do heaps exist? Set the level to Hard for the 61 cards that hurt most.

How were the answers checked?

Every answer was checked on 20 September 2026. In order of use, the sources are Wikipedia, NASA, the Stanford Encyclopedia of Philosophy, the U.S. Geological Survey, NOAA, the Smithsonian and the Natural History Museum. Nothing that changes from year to year is used.

Are these questions suitable for kids?

Yes, every card is clean. Set the level to Easy for 43 cards a ten-year-old can guess at, such as How many hearts beat inside one octopus? The Hard cards suit teens and adults, because the logic takes some holding in your head.

Do the logic knots have real answers?

Each one shows the standard resolution and, where one is known, credits its originator, from Zeno of Elea to Bertrand Russell. A few, such as the surprise quiz, are still argued over by philosophers, and the card says so.

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