Imagine four men buried up to their necks in the ground, as shown in the illustration. Between A and B is a solid brick wall. The men can only look forward, so A and B can see only the brick wall, C can see only B, and D can see both B and C.
The men know that among them are four hats: two black and two white. None of the men knows the colour of the hat on his own head.
To avoid being shot (I l-o-o-ove puzzles that include that phrase), any one of the men must correctly call out the colour of his own hat. If he gets it wrong, all four men will be shot. The men are not allowed to talk to each other and have 10 minutes to think.
After one minute, one of them calls out the correct colour of his own hat, thereby saving them all. I'd like to think that what happens next is that they are immediately dug up, relieved of their triangular hats, and taken to a Radiohead concert as a reward.
Questions for you:
1. Who calls out?
2. How does he know?
** I pinched the illustration from that site too.