Four explorers are captured by Impi Warriors and taken to their Cheif. The Chief ponders their fate and decides to offer them a lifeline if they can solve a puzzle. He shows them a table with two white hats and two black ones. He blindfolds them, and places a hat on each. Then, he puts one on one side of a wall, and the other three in a line on the other side, facing the wall:
A | B C D
A and B can see nothing but the wall. C can see B's hat. D can see B's hat and C's hat.
The Chief tells the four explorers that if one of them could stand up and tell the Chief the colour of his hat, they will be released.
ANSWER
If D sees two hats of the same colour, then he knows his hat is the opposite colour, so he will speak almost immediately. However, if after a reasonable interval, he does not speak, then C knows that D must be seeing two hats of opposite colour. Since C knows the colour of B's hat, he knows his hat is the other colour, and so he can correctly announce his hat colour.
