30 brain teasers for adults

A brain teaser is a puzzle that looks easy, has an obvious answer, and the obvious answer is wrong. The good ones need no maths beyond primary school and no facts at all. They just need you to slow down and read every word.

Here are 30 brain teasers for adults, with answers and a line on why each one works: ten logic puzzles, ten lateral thinking teasers and ten number teasers. Some are old favourites told our way, and the answers are folded under each set. Try them on your own, or read them out at the dinner table and watch who goes quiet.

• Logic puzzles
• Lateral thinking
• Number teasers
• A brain teaser every day

Logic brain teasers

1. Three light switches sit outside a closed room. One of them works the single lamp inside. You can flip the switches as much as you like, but you may open the door only once. How do you find the right switch?
2. A farmer needs to row a dingo, a chicken and a bag of grain across a river. The boat holds the farmer and one other thing. Left alone, the dingo eats the chicken and the chicken eats the grain. How does everything get across?
3. You have a 3 litre jug, a 5 litre jug and a tap. How do you measure exactly 4 litres?
4. Two doors: one leads out, one does not. A guard stands at each. One always lies and one always tells the truth, and you do not know which is which. You can ask one guard one question. What do you ask?
5. A drawer holds loose black socks and loose blue socks, and the room is dark. How many socks must you take out to be sure of a matching pair?
6. In a running race you overtake the person in second place. What place are you in now?
7. Ana is taller than Ben. Ben is taller than Cal. Cal is taller than Dee. Who is the second shortest?
8. A clock takes 5 seconds to strike six o'clock, timed from the first strike to the last. How long does it take to strike twelve?
9. Three boxes are labelled Apples, Oranges and Mixed, and every label is wrong. You may take one piece of fruit from one box without looking in. Which box do you pick from, and how do you fix all three labels?
10. A man looks at a photo and says, "Brothers and sisters I have none, but that man's father is my father's son." Who is in the photo?

Answers to the logic teasers

1. Turn on switch one and wait ten minutes. Turn it off, turn on switch two, then open the door. If the lamp is on, it is switch two. If it is off but warm, switch one. Off and cold, switch three.
2. Take the chicken over. Come back, take the dingo over, and bring the chicken back. Take the grain over and leave it with the dingo. Come back for the chicken.
3. Fill the 5 and pour it into the 3, which leaves 2 in the 5. Empty the 3 and pour the 2 into it. Fill the 5 again and top up the 3, which takes 1 litre. That leaves 4 in the 5.
4. Ask either guard, "Which door would the other guard say leads out?" Then take the other door. Both guards point you at the wrong one.
5. Three. With only two colours, any three socks must include two of the same colour.
6. Second. You took their place, not the leader's.
7. Cal. From tallest to shortest it goes Ana, Ben, Cal, Dee.
8. 11 seconds. Six strikes have five gaps, so each gap is one second. Twelve strikes have eleven gaps.
9. Take a fruit from the box labelled Mixed. It cannot be mixed, so if you pull out an apple it is the Apples box. The box labelled Oranges cannot hold oranges or apples, so it is Mixed, and the last box is Oranges. It works the same way if you pull out an orange.
10. His son. "My father's son" is the man himself, since he has no brothers, so the man in the photo is his son.

Lateral thinking brain teasers

1. How many months of the year have 28 days?
2. What can you hold in your right hand but never in your left?
3. Before Mount Everest was measured, what was the tallest mountain in the world?
4. A rooster lays an egg right on the peak of a roof. Which way does it roll?
5. How far can a dog run into a forest?
6. Which weighs more, a kilo of feathers or a kilo of bricks?
7. A doctor gives you three tablets and tells you to take one every half hour. How long do they last?
8. Divide 30 by a half and add 10. What do you get?
9. Two mothers and two daughters go fishing. They catch three fish, one each, and nobody goes without. How?
10. A woman shoots a photo of her husband, holds him under water for five minutes, then hangs him up to dry. They go out to dinner that night. How?

Answers to the lateral thinking teasers

1. All twelve. Every month has at least 28 days.
2. Your left hand, or your left elbow.
3. Mount Everest. It was the tallest whether anyone had measured it or not.
4. It doesn't. Roosters don't lay eggs.
5. Halfway. After that it is running out of the forest.
6. Neither. A kilo is a kilo.
7. An hour. One now, one at the half hour and one at the hour.
8. 70. Dividing by a half is the same as doubling, so 30 becomes 60.
9. There are only three of them: a grandmother, her daughter and her granddaughter.
10. She is a photographer developing a print in a darkroom: she shoots the photo, soaks the print in a developing tray, then hangs it up to dry.

Number brain teasers

1. A cricket bat and a ball cost $1.10 together. The bat costs $1 more than the ball. How much is the ball?
2. If 5 machines make 5 widgets in 5 minutes, how long do 100 machines take to make 100 widgets?
3. A patch of lily pads doubles in size every day. It covers the whole pond on day 48. On which day did it cover half the pond?
4. What comes next: 1, 1, 2, 3, 5, 8, ...?
5. What comes next: 2, 6, 12, 20, 30, ...?
6. How many times can you take 5 away from 25?
7. A brick weighs 1 kilo plus half a brick. How much does a whole brick weigh?
8. Think of a number. Multiply it by 3, add 6, divide by 3, then take away the number you first thought of. What do you get?
9. Sally has four brothers. Each brother has one sister. How many children are in the family?
10. A snail climbs 3 metres up a 10 metre wall every day and slides back 2 metres every night. How many days does it take to reach the top?

Answers to the number teasers

1. 5 cents. The bat is then $1.05, and together they make $1.10. The answer most people give, 10 cents, would make the total $1.20.
2. 5 minutes. Each machine takes 5 minutes to make one widget.
3. Day 47. It doubles to cover the whole pond the next day.
4. 13. Each number is the sum of the two before it.
5. 42. The gaps grow by two each time: 4, 6, 8, 10, then 12.
6. Once. After that you are taking 5 away from 20.
7. 2 kilos. If half a brick is 1 kilo, the whole brick is 2.
8. 2, whatever number you started with.
9. Five: four boys and Sally.
10. 8 days. After seven days and nights it is at 7 metres, and on day eight it climbs the last 3 and is over the top before it can slide.

A brain teaser every day, free

When you have run out of these, Keyhole Games sets new ones every day. There are eight free daily puzzles, each a different kind of thinking:

• Riddle of the day: three riddles, easy to hard, spun into a brass cylinder lock.
• Daily quiz: ten questions with the answers explained.
• Word grid: sixteen words hiding four groups of four.
• Whodunit: a small cast, a few clues and one culprit.
• Sliding block puzzle: clear a path and roll the payload out.
• Morse code puzzle: tune the radio, then decode the message.
• Geography puzzle: five coded clues about one real place.
• Today's puzzle: a code, a wheel or a workshop puzzle, different every day.

Each one is free, runs in your phone's browser and resets at midnight wherever you are. Every solve earns a key for your key ring, each daily with its own key, and your streak counts every day you finish, with one spare key to cover a missed day. Where a daily has hints, they are free and never cost your streak. Nothing to download and no account to make.

More to think about: our riddles with answers and trivia questions with answers.

Updated 28 October 2026.