The three switches
2/5
In a room there are three switches. In another room, which you can't see from the first, there is a light bulb connected to only one of them. You can manipul...
Visual traps
Visual and spatial traps that force you to question assumptions before concluding.
The boat and the ladder
1/5
A boat is moored to the dock. On its side there is a ladder with 10 steps visible above the water, separated by 20 cm. The tide rises 15 cm per hour. In 2 ho...
The round sewers
2/5
Many manhole covers are circular. They could have other flat shapes—square, rectangular, triangular, or oval—but the round shape repeats for some reason. Wha...
The clock and its hands
2/5
On an analog watch, how many times a day do the hour hand and minute hand matches exactly? Count each match only once.
The 100-story building and the two eggs
2/5
You have two identical eggs and access to a 100-story building. There is a critical floor such that: if an egg is dropped from that floor or from a higher on...
The rope alrededor of the Tierra
2/5
A rope surrounds the Earth adjusted to the equator. It is then lengthened exactly 2\pi meters and placed again forming a concentric circle, uniformly separat...
Mutilated board and dominoes
3/5
On a \(8\times 8\) chessboard, two opposite corners are eliminated. Is it possible to cover exactly the remaining 62 squares with \(1\times 2\) dominoes, wit...
The mosca between two trenes
2/5
Two trains are 300 km apart and moving towards each other, each at 60 km/h. A fly leaves the first train towards the second at 90 km/h. Every time it hits on...
The plane and the conveyor belt
2/5
An airplane is on a runway that is, in reality, a conveyor belt. The belt moves rearward and adjusts its speed to match the speed of the plane forward. The a...
The boat and the marbles in the lake
2/5
You are in a boat in the middle of a lake. Inside the boat is a box full of very heavy steel marbles. If you throw all the marbles into the water, does the l...
The square doble
2/5
You have a square drawn on a sheet. Someone challenges you to draw another square that has exactly twice the area, but with one condition: you cannot measure...