The thousand poisoned bottles
Level 5 · Expert · ●●●●●
You have 1000 bottles of wine. One of them is poisoned. One drop from the poisoned bottle kills a rat in exactly 24 hours. You have 10 lab rats. How much tim...
Chance and uncertainty
Riddles that sharpen probabilistic intuition and decision-making under uncertainty.
The hundred numbered boxes
Level 4 · Advanced · ●●●●○
There are 100 prisoners numbered 1 to 100. In a room there are 100 closed boxes, also numbered 1 to 100. Inside each box is a piece of paper with a number fr...
The hundred prisoners with hats
Level 4 · Advanced · ●●●●○
There are 100 prisoners in line. They randomly put a blue or red hat on each one. Each prisoner can see the hats of everyone in front of him, but not his own...
The light bulb room
Level 4 · Advanced · ●●●●○
There are 100 prisoners. Each day, the guard chooses one at random and takes it to a room with a light bulb (with on/off switch). The prisoner can flip the s...
The Monty Hall Problem
Level 4 · Advanced · ●●●●○
You are in a contest with three closed doors. Behind one there is a car (prize), behind the other two there are goats. You choose a door (let's say door 1)....
The envious dice
Level 4 · Advanced · ●●●●○
Four special dice: A: 4, 4, 4, 4, 0, 0 B: 3, 3, 3, 3, 3, 3 C: 6, 6, 2, 2, 2, 2 D: 5, 5, 5, 1, 1, 1 You choose a die first. The dealer then chooses another di...
The last passenger
Level 4 · Advanced · ●●●●○
An airplane has n seats ( n\ge 2 ) and n passengers with assigned seats. Passenger 1 loses his card and sits in a random seat. From passenger 2 onwards: if y...
White balls in two boxes
Level 4 · Advanced · ●●●●○
You have 50 white balls and 50 black balls. You must distribute them in two boxes, complying with: each box has at least one ball. Then this experiment is do...
The two-sided coin
Level 1 · Starter · ●○○○○
There are two coins in a bag: Coin A: has a face on both sides. Coin B: it is normal (heads and tails). You pull out a coin at random, flip it and it comes u...
The traffic light that reprograms itself
Level 1 · Starter · ●○○○○
An experimental traffic light has three lights: red ( R ), yellow ( A ), and green ( V ). Every minute, the state of the next minute is calculated like this:...
The urn bet
Level 5 · Expert · ●●●●●
Initial urn: 1 red and 1 blue ball. In each turn: a ball is drawn at random, is returned to the urn, and an extra ball of the same color is added. After n tu...
Las 100 monedas a ciegas
Level 3 · Intermediate · ●●●○○
Hay 100 monedas sobre una mesa. Sabes que exactamente 20 están cara arriba y 80 cara abajo, pero estás completamente a oscuras. Puedes separar las monedas en...
La última bola
Level 3 · Intermediate · ●●●○○
En una urna hay bolas blancas y negras. Repetidamente extraes dos bolas: si son del mismo color, retiras ambas y metes una negra; si son de distinto color, r...
Los cuatro prisioneros y el muro
Level 4 · Advanced · ●●●●○
Cuatro prisioneros (A, B, C y D) están alineados. Hay un muro entre C y D. A ve a B y C. B ve a C. C y D solo ven el muro. Se reparten 4 sombreros: 2 negros...