30 brain teasers for adults, with answers and explanations

Thirty brain teasers, sorted into numbers, words and logic, each with the answer hidden behind a click and an explanation of why it is right. None needs specialist knowledge. Most take under two minutes. Try them cold, then read the explanation even when you were right; the trick is usually more interesting than the answer.

Number teasers

A bat and a ball cost £1.10 together. The bat costs £1 more than the ball. How much is the ball?
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5p
If the ball were 10p, the bat would be £1.10 and the pair £1.20. Ball 5p, bat £1.05, total £1.10. Most people answer 10p on instinct; that is the point of the puzzle.
What is the next number: 2, 3, 5, 9, 17, ?
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33
The gaps double: +1, +2, +4, +8, so the next gap is +16. Equivalently, each term is double the last minus 1.
I am a three-digit number. My tens digit is five more than my ones digit, and my hundreds digit is eight less than my tens digit. What am I?
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194
Tens is five more than ones and at most 9, so ones is 4 or less and tens is 9 at most. Hundreds is tens minus 8, so tens must be 9 and hundreds 1. Ones is 4.
Using each of the digits 1 to 9 once, can three three-digit numbers add up to 1,000?
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No
The digits 1 to 9 sum to 45, which is divisible by 9. Any number made from them, and any sum of such numbers, is also divisible by 9. 1,000 is not.
A snail climbs a 10-metre wall. Each day it climbs 3 metres; each night it slips back 2. On which day does it reach the top?
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Day 8
It nets 1 metre a day, so after 7 days and nights it is at 7 metres. On day 8 it climbs 3 metres and reaches 10 before it can slip.
What comes next: 1, 11, 21, 1211, 111221, ?
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312211
Each term describes the previous one aloud. 111221 is “three 1s, two 2s, one 1”: 31 22 11.
You have 12 coins and a balance scale. One coin is heavier. What is the fewest weighings guaranteed to find it?
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3
Each weighing has three outcomes (left, right, balanced), so three weighings distinguish up to 27 cases. Split 4-4-4, weigh two groups, then 2-2 within the heavy four (or the unweighed four), then 1-1.
Multiply all the whole numbers from −5 to 5. What do you get?
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0
Zero is one of the factors. Everything times zero is zero. The negatives are a distraction.
How many times can you subtract 5 from 25?
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Once
After the first subtraction you are subtracting 5 from 20, not from 25.
A clock strikes six in five seconds. How long does it take to strike twelve?
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11 seconds
Six strikes have five gaps between them, so each gap is one second. Twelve strikes have eleven gaps.

Word teasers

Which English word becomes shorter when you add two letters to it?
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Short
Short plus “er” is shorter. The answer is hiding in the question’s own wording.
Rearrange the letters of LISTEN to make three other common words.
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Silent, enlist, tinsel
Inlets is a fourth. Pairing the vowels I and E with likely consonants gets you there quickly; see our anagram techniques.
What word is pronounced the same when four of its five letters are removed?
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Queue
Q on its own is pronounced “queue”. The remaining letters are silent passengers.
Forward I am heavy, backward I am not. What am I?
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Ton
Ton reversed is “not”. A reversal riddle: read the word backwards.
What five-letter word becomes a different word when you remove its first letter, and yet another word when you remove its first two letters?
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Stone
Stone, tone, one. Several words work (spine, pine, ine does not), which is why the puzzle usually specifies five letters and stops at two removals.
I have keys but no locks, space but no room, and you can enter but not go inside. What am I?
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A keyboard
Keys, the space bar, and the Enter key. Each clue is a literal feature described figuratively.
Which word in the dictionary is always spelled incorrectly?
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Incorrectly
The word “incorrectly” is spelled i-n-c-o-r-r-e-c-t-l-y. Read the question as a statement rather than a complaint.
What English word has three consecutive double letters?
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Bookkeeper
Oo, kk, ee. Bookkeeping has the same run. The letters are the clue: think of trades that involve books.
Take away my first letter and I sound the same. Take away my last letter and I still sound the same. Even take away my middle letter and I sound the same. What am I?
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Empty
Empty, mpty (M-T), empt (M-T), and emty all say “em-tee”. Say the letters aloud rather than reading them.
What is the longest common English word you can type using only the top row of a keyboard?
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Typewriter
Q W E R T Y U I O P contains every letter of typewriter. Proprietor and repertoire also work, with ten and ten letters.

Logic teasers

A man looks at a portrait and says: “Brothers and sisters I have none, but that man’s father is my father’s son.” Who is in the portrait?
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His son
“My father’s son” with no brothers is the speaker himself. So the man’s father is the speaker, and the man is his son.
You are in a room with three light switches. Each controls one of three bulbs in another room. You may flip switches as you like but can only visit the other room once. How do you match them?
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Use heat
Turn on switch A for ten minutes, then turn it off and turn on B. Visit: the lit bulb is B, the warm dark bulb is A, the cold dark bulb is C.
Two fathers and two sons go fishing. Each catches one fish, yet they bring home three fish. How?
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There are three people
A grandfather, his son and his grandson. The middle man is both a father and a son.
If it takes five machines five minutes to make five widgets, how long do 100 machines take to make 100 widgets?
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5 minutes
Each machine makes one widget in five minutes. A hundred machines working at once make a hundred widgets in the same five minutes.
A woman has two coins totalling 60p. One of them is not a 50p. What are the coins?
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A 50p and a 10p
One coin is not a 50p: the 10p. The other one is. The puzzle relies on you hearing “neither” when it says “one”.
Three boxes are labelled Apples, Oranges and Mixed. Every label is wrong. You may take one fruit from one box without looking inside. How do you relabel all three correctly?
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Draw from “Mixed”
Because every label is wrong, the Mixed box holds only apples or only oranges. Whatever you draw names that box. The box labelled with that fruit cannot hold it and cannot be Mixed’s true contents, so it is the other single fruit, and the last box is Mixed.
A farmer has 17 sheep. All but 9 die. How many are left?
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9
“All but nine” means nine survive. The temptation is to subtract.
You are running a race and overtake the person in second place. What place are you in now?
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Second
You took their place. To be first you would have to overtake the leader.
What has cities but no houses, forests but no trees, and water but no fish?
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A map
Each feature is present as a symbol, not the thing itself.
Ten people meet and each shakes hands with everyone else exactly once. How many handshakes?
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45
Each of 10 people shakes 9 hands, giving 90, but each handshake has been counted twice. 90 ÷ 2 = 45. It is also the ninth triangular number.

Want more?

The number puzzles here are the kind Brisk serves two of every day as its brain teaser of the day, and the sequence method covers the checklist that cracks them. For a page of arithmetic puzzles only, see math brain teasers. For what puzzles like these do and do not do for your mind, read do brain games work?

Common questions

What makes a good brain teaser for adults?

A short statement, one clean trick, and an answer you could have reached with what you were given. The best ones exploit a habit of thought (subtracting instead of reading, hearing “neither” for “one”) rather than obscure knowledge.

Are brain teasers good for your brain?

They are enjoyable and briefly demanding, and regular puzzling is associated with better thinking skills in large surveys of older adults. No study shows teasers prevent decline or raise intelligence. We set out the evidence honestly in our research section.

Can I get a new brain teaser every day?

Yes. Brisk publishes two pattern puzzles a day as its brain teaser of the day, inside a six-question challenge that also covers maths and words.