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Logic, math, everyday and lateral-thinking riddles — each one paired with a step-by-step visual walkthrough of the solution, right in your browser.
Ferry a wolf, a goat and a cabbage across a river without ever leaving a predator alone with its prey.
On an island of truth-tellers and liars, one islander's claim is enough to deduce everyone's nature.
Two opposite corners are cut off. Can 31 dominoes still tile the board?
One of three switches controls a hidden bulb. Identify it with a single visit to the room.
Fill a 3×3 grid so each row and column contains 1, 2 and 3 — pure elimination.
Three piles of stones and a winning strategy hidden in binary XOR.
One guard always lies, one always tells the truth. Find the safe door with a single question.
Three boxes, every label wrong. Fix all of them by drawing just one fruit.
Two friends know only the month or the day. Their conversation reveals the date.
A visitor says something everyone already knew. Three days later, three people leave.
The majority prefers A to B, B to C, and C to A. Who should win?
Five houses, fifteen clues, one question: who owns the zebra?
Move a stack of disks to another peg, one at a time, never resting a larger disk on a smaller one.
Spot the hidden rule behind 2, 6, 12, 20, 30 and predict the next term.
A bat and ball cost $1.10; the bat costs $1 more. The obvious answer is wrong.
Add 1 to 100 in seconds — the trick a young Gauss reputedly found in class.
One grain on the first square, doubling each square. How much rice by the 64th?
If everyone in a room of six shakes hands once, how many handshakes happen?
1, 1, 2, 3, 5, 8, 13 … the sequence that models breeding rabbits — and appears everywhere.
Why does 1 + 3 + 5 + 7 + … always land on a perfect square? See it, don't prove it.
$27 paid, $2 kept — that's $29. So where is the thirtieth dollar?
Five doublings in a row, then the pattern breaks. The answer is not 32.
The sequence that counts binary trees, balanced brackets and polygon cuts — all at once.
Stand in a circle, count off every second person. Where do you stand to be the last one left?
Measure exactly four litres with only a five-litre and a three-litre jug.
Get four people with different speeds across a night bridge, sharing one torch, in the least time.
Two ropes, each burning an hour but unevenly. Measure exactly 45 minutes.
Blue and black socks jumbled in a drawer. How many grabs guarantee a matching pair?
A camel hauls bananas to market but eats them as it walks. How many survive the trip?
One of eight bottles is poisoned. Three tasters and binary code find it in one day.
Find the highest safe floor in a 100-storey building with just two eggs — in as few drops as possible.
100 lockers, 100 passes of toggling. Which ones end up open?
A monk climbs up one day and down the next. Is there a spot he passes at the same time both days?
Ants bounce off each other on a 1 m stick. How long until the last one falls off?
Interview once, decide on the spot, no second chances. When should you stop looking?
Add one metre to a rope circling the equator. How high does it float?
One of eight coins is heavier. Pin it down with only two turns of a balance scale.
A game show, three doors, and a counter-intuitive truth: switching doubles your odds.
Five ruthless pirates split 100 coins by vote. How much can the captain keep?
Scale the counterfeit-coin hunt to twelve coins — still just three weighings.
A line of prisoners in red or blue hats. One clever signal saves almost everyone.
How few people does it take for two to share a birthday? Far fewer than you'd think.
Four cards, one logical rule, and the classic trap of confirming instead of falsifying.
A family has two kids, at least one a boy. The chance both are boys isn't ½.
Two suspects, one choice each. Why rational self-interest lands both in worse cells.
Guessing gives one chance in a thousand. One clever rule lifts it to one in three.
Your test came back positive and the test is 95% accurate. Should you panic?
Treatment A wins for small stones and for large stones — and loses overall.