A new white bean is added to the bag, and it is shaken. A bean is taken back out, and it is white. A farmer claims to have 4 trees on his land all equidistant from each other. In a far away land, there is an unusual tribe of 300 perfectly logical and perfectly intelligent people. Each member has a visible spot on the back of his or her head, some are red and some are black.
Nobody knows the color of their own spot, but they do know the color of everyone else's.
If a tribesman ever realizes the color of his own spot it is strict custom that he publicly commit suicide the following morning, so they never mention spot colors, and have no mirrors. The tourist leaves and returns a year later. One Question You are shipwrecked on an island. There is a fork in the path to the other side, one way leads to a safe village, the other leads to hungry cannibals. There are twin brothers who both know which path is which, but one of the brothers is honest, and the other always lies.