Brain-Teaser Puzzles to Challenge Your Mind

Brain teasers and logic puzzles are a fun way to exercise your mind. In fact, research shows that regularly solving puzzles “reinforces connections between brain cells, improves mental speed and is an effective way to improve short-term memory.”.

Puzzles also “sharpen your logic and reasoning” and hone critical thinking. Below is a curated mix of ten challenging puzzles for adults – from word riddles to logic and math problems. Each puzzle is followed by its answer and a brief explanation to help you understand the solution. Enjoy the challenge and see if you can figure them out before reading the answer!

Riddles and Word Puzzles

• Puzzle (Word Riddle): Peter’s father has five sons. The names of four sons are Fefe, Fifi, Fafa and Fufu. What is the name of the fifth son?
  Answer: The fifth son is Peter himself. The riddle cleverly hides the answer in the question: “Peter’s father has five sons,” which means one of them is Peter.

• Puzzle (Riddle): “You bury me when I am alive, you dig me up when I die. What am I?”
  Answer: A plant. When a plant is alive, we sow or bury its seeds; when it dies (e.g. a flower wilts), we often pull it up or dig it out. This classic “what am I” riddle’s solution is a plant.

• Puzzle (Riddle): “What can you catch but not throw?”
  Answer: A cold. You can “catch a cold,” but of course you can’t literally throw it. The answer is the common viral illness, a cold.

Logic Puzzles

• Puzzle (Logic): A meal costs €97. You pay with a €100 bill, the waiter returns €3 in change, and you also leave a €2 tip. The way the total is often calculated seems confusing – what is your actual expense?
  Answer: You actually spent €98 in total. Here’s why: the meal was €97 and your €2 tip makes €99 spent, but you received €3 change, so net you paid €98. In other words, €97 for the meal plus €1 (of the original €100) as tip equals €98 total.

• Puzzle (Logic): You have two ropes and a lighter. Each rope burns completely in exactly 1 hour, but they do not burn at a steady rate (one half might burn quickly and the other half slowly). How can you measure exactly 45 minutes?
  Answer: Light both ends of the first rope and one end of the second rope at the same time. The first rope will burn out in 30 minutes (because burning both ends halves the time). At that moment 30 minutes have passed. Then light the other end of the remaining rope – it now has 30 minutes’ worth of burn remaining but burning from both ends will take 15 minutes. 30+15 = 45 minutes.

• Puzzle (Logic): You have 9 identical balls, but one ball is heavier than the others. You have a balance scale and are allowed only two weighings. How can you find the heavier ball?
  Answer: Divide the balls into three groups of 3. First weigh any two groups of 3 against each other. If they balance, the heavy ball is in the third group; if not, the heavier side contains the heavy ball. Then take the three-ball group containing the odd ball and weigh two of those balls against each other. If one is heavier, that’s the odd ball; if they balance, the ball not on the scale is the heavier one.

• Puzzle (Logic): You are outside a closed room with three light switches. In the room there is a single light bulb (currently off) that is controlled by exactly one of the three switches. You may flip the switches as you like, but you can only enter the room once to check the light. How can you determine which switch controls the bulb?
  Answer: First, turn on two switches and leave them on for a few minutes, then turn one of those two off. Now enter the room. If the bulb is on, it’s controlled by the switch you left on. If the bulb is off but warm, it’s controlled by the switch you turned off (it was on long enough to heat up). If the bulb is off and cold, it’s controlled by the third switch you never turned on.

• Puzzle (Logic/Lateral): You’re in a room with two doors: one leads to freedom and one leads to doom. Two guards know which door is which – one guard always tells the truth, and the other always lies. You may ask exactly one yes/no question to one guard to figure out the safe door. What do you ask, and which door do you choose?
  Answer: Ask either guard: “If I asked the other guard which door leads to freedom, what would he say?” Then choose the opposite door. Why it works: the truthful guard would truthfully report the liar’s answer, and the liar would lie about the truthful guard’s answer. In either case, they point to the wrong door, so the opposite door is safe.

• Puzzle (Logic): A farmer needs to ferry a wolf, a goat, and a cabbage across a river using a boat that can carry only him plus one item. If left alone together, the wolf will eat the goat, and the goat will eat the cabbage. How can he transport all three safely?
  Answer: First, take the goat across and leave it on the other side. Return alone. Next, take the wolf across. Then bring the goat back to the original side. Leave the goat, and take the cabbage across. Leave the cabbage with the wolf, then finally return alone to fetch the goat. The sequence is Goat – Return – Wolf – Goat back – Cabbage – Return Goat. This way, the goat is never left alone with the wolf or the cabbage unprotected.

Math and Probability Puzzles

• Puzzle (Math): A fish’s head is 3 cm long. Its tail is as long as the head plus half the length of the body. The body is as long as the head and tail together. How long is the entire fish?
  Answer: The fish is 24 cm long. You can solve by setting up relationships or by logical trial. In fact, the breakdown is: head = 3 cm, body = 12 cm, tail = 9 cm, which adds to 24 cm.

• Puzzle (Probability): You’re on a game show with three doors. Behind one door is a car; behind the other two doors are goats. You pick one door (say Door 1). The host, who knows what’s behind each door, then opens another door (say Door 3) revealing a goat. He then offers you the chance to switch your choice to the remaining unopened door (Door 2). Should you switch or stay, and why?
  Answer: You should switch. Counterintuitively, switching doubles your chances of winning from 1/3 to 2/3. Initially you had a 1/3 chance of choosing the car. After the host reveals a goat behind another door, the probability that the car is behind the other unopened door becomes 2/3, while your original pick still has only 1/3. In other words, switching gives you a 2/3 probability of winning.

Lateral Thinking Puzzles

• Puzzle (Lateral): A man lives on the 10th floor of a building. Every day he rides the elevator down to go to work or shopping. But when he returns, he rides the elevator only to the 7th floor and walks up the stairs the rest of the way (to the 10th floor), unless it’s raining. Why?
  Answer: The man is short. He can only reach the elevator buttons up to the 7th floor. On dry days, he must get off at 7 and walk up. On rainy days, he has an umbrella which he can use to press the 10th-floor button. In short, the umbrella extends his reach.

• Puzzle (Lateral/Math): Can you draw a polygon with two sides and two right angles? (A “polygon” here means a closed shape in the usual sense.)
  Answer: Yes – on the surface of a sphere. On a flat plane a two-sided polygon is impossible, but on a sphere you can draw a shape with two edges (great circle segments) and two 90° angles, for example by drawing two perpendicular semicircles that share endpoints at the poles.

• Puzzle (Lateral): Five glasses are in a row: the first three contain water and the last two contain poison. By moving exactly one glass (without mixing liquids), arrange the glasses so that no two water glasses are adjacent. What do you do?
  Answer: Solution: Pour the water from the second glass into the fourth glass (which had poison), emptying glass 2. Then place the now-empty glass in the second position in the row. The final order will alternate water/poison such that no two waters are adjacent. (In effect, you move glass 2, by emptying it and placing it in the middle.)

Each puzzle above illustrates a different kind of critical thinking. Take your time and see if you can solve them on your own before checking the answers. Happy puzzling – and keep your mind sharp!