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Printable riddle sheet — Ages 18+

56 riddles, ready to print — with the answer key on its own page.

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Ages 18+ — The Deep End riddle sheet

56 riddles across 4 categories · write your guess on the line, then check the answer key at the end.

Wordplay

  1. 1. Start with the word STARTLING. Remove exactly one letter at a time, and after every single removal, what remains must still be a real English word — all the way down to one letter. What's the sequence?

    My guess: ______________________________________________

  2. 2. What five-letter palindrome (reads the same forwards and backwards) names a small boat you paddle?

    My guess: ______________________________________________

  3. 3. Rearrange the letters of the word LISTEN to form another common word — one that describes paying quiet attention.

    My guess: ______________________________________________

  4. 4. What single word describes both a financial institution and the sloped land beside a river?

    My guess: ______________________________________________

  5. 5. What seven-letter word — spelled T-H-E-R-E-I-N — contains, without skipping or rearranging any letters, all of these five smaller words hidden inside it: 'the,' 'he,' 'her,' 'here,' and 'rein'?

    My guess: ______________________________________________

  6. 6. What word famously means the exact same thing whether or not you add the prefix 'in-' to the front of it — to the eternal confusion of firefighters?

    My guess: ______________________________________________

  7. 7. What is the only number from one to a hundred whose letters, spelled out in English, appear in strict alphabetical order?

    My guess: ______________________________________________

  8. 8. What is the longest common English word that can be typed using only the left hand on a standard QWERTY keyboard?

    My guess: ______________________________________________

  9. 9. What is the only common English word that ends in the letters '-mt'?

    My guess: ______________________________________________

  10. 10. Barely any English words contain a double U ('uu'). Name the everyday one that means a space with all the matter taken out of it — or the machine named after that idea.

    My guess: ______________________________________________

  11. 11. What four-letter word reads the same backwards as forwards — and also reads the same when you rotate the page a half turn?

    My guess: ______________________________________________

  12. 12. One eight-letter word for being under pressure becomes, spelled backwards, the sweet course at the end of a meal. What are the two words?

    My guess: ______________________________________________

  13. 13. Which English verb is its own opposite, meaning both 'to split violently apart' and 'to cling faithfully to'?

    My guess: ______________________________________________

  14. 14. Which everyday word means 'to rub something to a shine' in lowercase, but names a nationality the moment you capitalise its first letter?

    My guess: ______________________________________________

Math & Logic

  1. 15. Three switches outside a windowless room each control one of three light bulbs inside. You can flip the switches as many times as you like, but you may only enter the room once. How do you determine, with total certainty, which switch controls which bulb?

    My guess: ______________________________________________

  2. 16. You have two identical eggs and a 100-floor building. You want to find the highest floor an egg can be dropped from without breaking, using as few worst-case drops as possible. What's the optimal worst-case number of drops?

    My guess: ______________________________________________

  3. 17. A man must cross a river carrying a wolf, a goat, and a bag of cabbage, one at a time in a small boat. Left alone, the wolf eats the goat, and the goat eats the cabbage. How does he get everything across safely?

    My guess: ______________________________________________

  4. 18. What comes next in this sequence, and why: 1, 11, 21, 1211, 111221, ?

    My guess: ______________________________________________

  5. 19. You have 12 identical-looking coins, and exactly one is counterfeit — either heavier or lighter than the rest, but you don't know which. Using a balance scale, what's the minimum number of weighings needed to both identify the counterfeit coin and determine whether it's heavier or lighter?

    My guess: ______________________________________________

  6. 20. Four people need to cross a rickety bridge at night, sharing one flashlight that must be carried on every crossing. The bridge holds only two people at a time, and a pair moves at the slower person's pace. Crossing times are 1, 2, 5, and 10 minutes. What is the minimum total time for everyone to cross?

    My guess: ______________________________________________

  7. 21. If it takes 5 people 5 hours to paint 5 identical fences, how long does it take 10 people to paint 10 identical fences, at the same steady rate?

    My guess: ______________________________________________

  8. 22. A father is currently 4 times as old as his son. In 20 years, he'll be exactly twice as old as his son. How old are they now?

    My guess: ______________________________________________

  9. 23. You reach a fork in the road: one path leads to the city, the other to certain ruin. Two guards stand there — one always tells the truth, one always lies, and you cannot tell which is which. You may ask exactly one yes-or-no question, to exactly one guard. What do you ask?

    My guess: ______________________________________________

  10. 24. Twenty-three people are picked at random and put in one room. Roughly what is the probability that at least two of them share a birthday?

    My guess: ______________________________________________

  11. 25. A drawer in a pitch-black room holds 10 black socks and 10 blue socks, identical except in colour. How many socks must you take out to be certain of a matching pair — and how many to be certain of a black pair?

    My guess: ______________________________________________

  12. 26. Imagine a rope tied snugly around the Earth's equator. You add exactly one metre to its length, then lift it so it stands the same height above the ground the whole way round. Roughly how high off the ground is it?

    My guess: ______________________________________________

  13. 27. A pile of 21 stones sits between two players. They take turns removing 1, 2, or 3 stones, and whoever takes the last stone wins. Would you rather move first or second, and what is the strategy?

    My guess: ______________________________________________

  14. 28. A disease affects 1 person in 10,000. A screening test catches 99% of the people who have it, and wrongly flags 1% of the people who don't. You test positive, with no symptoms and no risk factors. Roughly what is the chance you actually have it?

    My guess: ______________________________________________

Lateral Thinking

  1. 29. A man is found dead in an open field, holding an unopened package. There are no other footprints anywhere nearby, and he clearly died from a fall — but there's no cliff, building, or ledge anywhere around. How did he die?

    My guess: ______________________________________________

  2. 30. A woman gives birth to two sons, born in the same hour of the same day of the same year, to the same two biological parents — yet they are not twins. How is that possible?

    My guess: ______________________________________________

  3. 31. You're on a game show facing three closed doors: a car behind one, goats behind the other two. You pick door #1. The host, who knows what's behind every door, opens door #3 to reveal a goat, then offers you the choice to switch to door #2 or stay with door #1. To maximize your odds of winning the car, what should you do, and what are your odds?

    My guess: ______________________________________________

  4. 32. A woman living in a small town has legally married ten different men from that same town. None of them died, none of them divorced her, and she broke no law. How is that possible?

    My guess: ______________________________________________

  5. 33. A man is walking the wrong way down a one-way street, and passes at least ten police officers along the way. None of them stop him or say a word. Why not?

    My guess: ______________________________________________

  6. 34. A man drinks a glass of punch at a party and leaves early. Later that night, everyone else who drank the punch becomes seriously ill. He's completely fine. Why didn't he get sick too?

    My guess: ______________________________________________

  7. 35. A woman claims she can predict the exact final score of any football (soccer) match before it even kicks off, every single time, without fail. How is this possible?

    My guess: ______________________________________________

  8. 36. What can go up a chimney down, but cannot go down a chimney up?

    My guess: ______________________________________________

  9. 37. A warship's captain radios a light dead ahead on a collision course and orders it to alter course 15 degrees north. Back comes: 'Alter YOUR course 15 degrees south.' Furious, the captain radios that he commands the largest ship in the fleet and demands they give way. The reply: 'I am a lighthouse.' Who moves?

    My guess: ______________________________________________

  10. 38. A man born in Portugal, who has lived there his entire life, cannot legally be buried anywhere in Spain — whatever his will says, and however much his family is willing to pay. Why not?

    My guess: ______________________________________________

  11. 39. A man needs to cross a long bridge into the next country. A guard steps out of his hut every five minutes to check the bridge, and orders anyone he sees back the way they came. The crossing takes eight minutes on foot, and there is no other route. How does he get across?

    My guess: ______________________________________________

  12. 40. A window cleaner falls from a window on the thirtieth floor of an office tower and walks away without a scratch. There was no net, no awning, and nothing broke his fall. How?

    My guess: ______________________________________________

  13. 41. A dying king tells his two sons that whichever son's camel reaches the distant city LAST will inherit the kingdom. The brothers wander the desert for weeks, each refusing to get ahead of the other. Finally a passing sage whispers two words, and both leap into the saddle and gallop off at full speed. What did the sage say?

    My guess: ______________________________________________

  14. 42. Manhole covers are round almost everywhere in the world. There is one practical reason squares, rectangles, and triangles all fail on. What is it?

    My guess: ______________________________________________

Trivia

  1. 43. In which century did Johannes Gutenberg's printing press revolutionize the spread of the written word in Europe?

    My guess: ______________________________________________

  2. 44. What is the only mammal capable of true, sustained flight?

    My guess: ______________________________________________

  3. 45. Which element has the chemical symbol 'Fe'?

    My guess: ______________________________________________

  4. 46. What is the collective noun for a group of crows?

    My guess: ______________________________________________

  5. 47. Who painted the ceiling of the Sistine Chapel?

    My guess: ______________________________________________

  6. 48. What is the smallest prime number?

    My guess: ______________________________________________

  7. 49. What economic term describes a situation where prices keep rising while economic growth stalls and unemployment stays high?

    My guess: ______________________________________________

  8. 50. Besides echidnas, what is the only other living mammal that lays eggs?

    My guess: ______________________________________________

  9. 51. What is the scientific study of fungi called?

    My guess: ______________________________________________

  10. 52. Einstein's Nobel Prize in Physics was not awarded for relativity. What was it awarded for?

    My guess: ______________________________________________

  11. 53. "Laser" began life as an acronym. What do its five letters stand for?

    My guess: ______________________________________________

  12. 54. Which is the only sea on Earth with no coastline, bounded entirely by ocean currents rather than by land?

    My guess: ______________________________________________

  13. 55. Which nineteenth-century novel opens with the three words "Call me Ishmael"?

    My guess: ______________________________________________

  14. 56. Which blood type is known as the universal donor for red blood cell transfusions?

    My guess: ______________________________________________

Answer key — Ages 18+

Wordplay

  1. 1. STARTLING → STARTING → STARING → STRING → STING → SING → SIN → IN → I — Each step removes one letter (L, then T, then A, then R, then T, then G, then S, then N) and every intermediate result is a valid word.
  2. 2. Kayak
  3. 3. Silent — LISTEN and SILENT are anagrams of each other, sharing all six letters.
  4. 4. Bank
  5. 5. Therein — T-HE-R-E-I-N: THE (1-3), HE (2-3), HER (2-4), HERE (2-5), and REIN (4-7) are all contiguous substrings.
  6. 6. Flammable / inflammable — Both words mean "capable of catching fire," a genuine quirk of English.
  7. 7. Forty — F-O-R-T-Y: F, O, R, T, Y fall in strictly ascending alphabetical order — no other spelled-out number does.
  8. 8. Stewardesses — Every letter in "stewardesses" (S, T, E, W, A, R, D) falls on the left side of a QWERTY keyboard.
  9. 9. Dreamt — Only its own compounds — "undreamt," "daydreamt" — share the ending.
  10. 10. Vacuum — "Continuum" and "residuum" are the other well-known ones; English almost always separates two u's with a consonant.
  11. 11. NOON — Reversed it is still NOON, and turned 180° each letter maps onto itself: N→N, O→O.
  12. 12. Stressed and desserts — STRESSED reversed is DESSERTS — a "semordnilap," a word that spells a different word backwards (as opposed to a palindrome, which spells itself).
  13. 13. Cleave — Two unrelated Old English verbs collapsed into one spelling. Words that contradict themselves like this are contronyms — "sanction" and "dust" are others.
  14. 14. polish / Polish — A capitonym — the capital letter changes the pronunciation too ("PAH-lish" versus "POH-lish").

Math & Logic

  1. 15. Turn switch 1 on for several minutes, then turn it off. Turn switch 2 on and immediately enter the room. The lit bulb is switch 2's. The off-but-warm bulb is switch 1's. The off-and-cool bulb is switch 3's.
  2. 16. 14 drops — Using a decreasing-interval strategy (start at floor 14, then 27, 39...), the worst case is bounded by the smallest n where n(n+1)/2 ≥ 100, which is n = 14.
  3. 17. Take the goat across first and return alone. Take the wolf across, bring the goat back. Take the cabbage across and return alone. Finally, take the goat across. — The goat crosses twice, which is the key — it's never left alone with the wolf or the cabbage.
  4. 18. 312211 — Each term describes the previous one aloud ("look-and-say"): 111221 is "three 1s, two 2s, one 1," which becomes 312211.
  5. 19. 3 weighings
  6. 20. 17 minutes — Send the two fastest (1+2) across together, send the fastest back, send the two slowest (5+10) across together, send the second-fastest back, then send the two fastest across again: 2+1+10+2+2 = 17.
  7. 21. 5 hours — The ratio of people to fences stays 1:1, so each person still needs the same 5 hours to finish their own fence.
  8. 22. The father is 40 and the son is 10. — f = 4s and f + 20 = 2(s + 20) → 4s + 20 = 2s + 40 → s = 10, f = 40. Check: in 20 years, 60 = 2 × 30.
  9. 23. Ask either guard: "If I asked the other guard whether the left path leads to the city, would he say yes?" Then take the opposite of the answer you get. — Either the truth-teller faithfully reports the liar's lie, or the liar misreports the truth-teller's honest answer — either way the reply is inverted exactly once, so it is reliably wrong. Reliably wrong is as useful as reliably right.
  10. 24. About 50% — 50.7%, to be exact — What matters is not "you against everyone else" but the 253 possible pairs among 23 people. Multiplying 365/365 × 364/365 × … × 343/365 gives about a 49.3% chance of no shared birthday at all.
  11. 25. Three for a matching pair; twelve for a black pair. — Three socks can only be two colours, so two of them must match. But the worst case for black is drawing all 10 blue socks first, then 2 black ones — 12 in total.
  12. 26. About 16 centimetres — enough to slide your hand under. — Circumference is 2πr, so adding 1 m of rope raises the radius by 1 ÷ 2π ≈ 0.159 m. The size of the Earth cancels out completely — the answer is identical for a rope around a tennis ball.
  13. 27. Move first: take 1 stone, then always take 4 minus whatever your opponent just took. — Taking 1 leaves 20, a multiple of 4. From then on each full round removes exactly 4 stones, so your opponent always faces a multiple of 4 and you take the last stone. Any pile that is a multiple of 4 loses for whoever has to move.
  14. 28. About 1% — Picture 10,000 people: about 1 has the disease and tests positive, while about 100 healthy people also test positive. That is roughly 1 true positive per 101 positives. Ignoring how rare the disease is — the "base rate" — is the classic error here.

Lateral Thinking

  1. 29. His parachute failed to open — he was skydiving.
  2. 30. They're two of a set of triplets (or more) — there's at least one other sibling from the same birth.
  3. 31. Switch to door #2 — doing so wins the car 2/3 of the time, versus only 1/3 if you stay. — The host's guaranteed reveal of a goat concentrates the original 2/3 chance that "the car is behind one of the other two doors" entirely onto the one door he didn't open. This is the classic Monty Hall problem.
  4. 32. She's a wedding officiant (like a minister or judge) — she "married" them by performing their weddings, not by being their wife.
  5. 33. He's walking, not driving — going the wrong way on foot down a one-way street isn't against the law, only driving is.
  6. 34. The poison was in the ice cubes, which hadn't melted yet when he drank his glass — by the time everyone else drank later, the ice had melted and poisoned their punch.
  7. 35. She always predicts 0–0 — which is genuinely, always the score before a match starts.
  8. 36. An umbrella — A closed ("down") umbrella fits up a chimney; an open ("up") one does not fit down a chimney.
  9. 37. The ship. A lighthouse is a fixed structure standing on rock — no amount of rank makes it move.
  10. 38. He is still alive.
  11. 39. He walks four minutes out, then turns around and walks back toward the side he started from. The guard sees him heading that way, assumes he came from the far country, and orders him back — across the bridge.
  12. 40. He was cleaning the inside of the window — he fell about a metre, into the room.
  13. 41. "Swap camels." — Once each brother is riding the other's camel, racing to arrive first makes his own camel — the one that decides the inheritance — arrive last.
  14. 42. A round cover cannot fall through its own hole. A square one, turned on its diagonal, drops straight down the shaft onto whoever is working below. — A circle has the same width in every direction; a square's diagonal is about 1.41 times its side, so it fits through the opening it just covered.

Trivia

  1. 43. The 15th century — Gutenberg's press dates to around 1440.
  2. 44. The bat
  3. 45. Iron — From the Latin word "ferrum."
  4. 46. A murder (of crows)
  5. 47. Michelangelo
  6. 48. 2 — It's the only even prime — every other even number is divisible by 2.
  7. 49. Stagflation
  8. 50. The platypus
  9. 51. Mycology
  10. 52. His explanation of the photoelectric effect — The 1921 prize cited "his discovery of the law of the photoelectric effect" — relativity was still considered too contentious to honour.
  11. 53. Light Amplification by Stimulated Emission of Radiation
  12. 54. The Sargasso Sea — It sits in the North Atlantic, held in place by the currents of the North Atlantic Gyre.
  13. 55. Moby-Dick, by Herman Melville — Published in 1851.
  14. 56. O negative — O-negative red cells carry neither A, B, nor RhD antigens, so a recipient's immune system has nothing to react against.

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