The three switches
Level 1 · Starter · ●○○○○
In a room there are three switches. In another room (which you can't see from where the switches are) there is a light bulb. You can only enter the light bul...
Visual traps
Visual and spatial traps that force you to question assumptions before concluding.
The boat and the ladder
Level 1 · Starter · ●○○○○
A ship is anchored in the port. On its side there is a staircase with 10 steps visible over the water. Each step is 20 cm high. The tide rises at a rate of 1...
The bridge crossing
Level 1 · Starter · ●○○○○
Four people need to cross a bridge at night. They only have one flashlight and the bridge can only hold two people at a time. Ana takes 1 minute to cross, Be...
The three prisoners and the red hats
Level 3 · Intermediate · ●●●○○
Three perfectly logical prisoners are in separate cells. The guard tells them, "I'll give each of you a red or blue hat. At least one hat is red. You can see...
Two prisoners, 64 coins and a secret pocket
Level 4 · Advanced · ●●●●○
There are two prisoners and a guard. The guard has a 8\times 8 chess board and a coin on each square (64 in total), each one heads or tails. Challenge rules:...
The two ropes
Level 3 · Intermediate · ●●●○○
You have two ropes and a lighter. Each rope takes exactly 1 hour to burn completely, but they do NOT burn evenly (some parts burn faster than others). How ca...
The nine points
Level 3 · Intermediate · ●●●○○
Draw 9 points forming a square of 3 \times 3 . Can you connect the 9 dots using only 4 continuous straight lines without lifting the pencil from the paper?
The tournament without a referee
Level 4 · Advanced · ●●●●○
In a round robin tournament with n players: each pair plays exactly once, there are no ties. Can you always order the players in a row P1,P2,\dots,Pn such th...
The tournament of ties
Level 4 · Advanced · ●●●●○
Five teams play a round-robin league, a single round. Rules: victory: 3 points, tie: 1 point per team, defeat: 0 points. In the end, the five teams end up ti...
The binary bells
Level 4 · Advanced · ●●●●○
Four bells A, B, C, D begin silently. There are four strings and each one changes (turns on/off) these bells: X changes A and B, And change B and C, Z change...
Mutilated board and dominoes
Level 5 · Expert · ●●●●●
On an 8x8 chess board you remove two opposite corners. Can the rest of the board be exactly covered with 1x2 dominoes, without overlapping or leaving gaps?
Dashboard infection
Level 5 · Expert · ●●●●●
On a 100\times100 board, exactly 99 squares start out infected. Minute rule: a healthy square is infected if it shares an orthogonal border with at least two...
Unique messenger route (graphs)
Level 3 · Intermediate · ●●●○○
A courier must travel each street exactly once on the following map: To avoid ambiguity, the streets (edges) are exactly these 9: A-B, A-C, B-C, B-D, C-D, C-...
The fly between two trains
Level 3 · Intermediate · ●●●○○
Two trains are 300 km apart and are approaching each other, each at 60 km/h. A fly leaves the first train towards the second at 90 km/h. When you touch the s...
12-step ladder
Level 3 · Intermediate · ●●●○○
You climb a 12-step ladder. In each movement you can go up 1 or 2 steps. How many different ways can you get to the top?