Originally posted by Jirakon
Here's one similar to yours, but a little more difficult (and don't answer if you've heard this one before):
An arbiter place a hat on the heads of three men. He tells them that all hats are either white or red. None of the men know at this point what color hat they are wearing, but they could see the color of the hats the other two men. The arbiter then ...[text shortened]... guessed what color his hat was. How did he know, and what colors were each of the three hats?
The hats must all be the same color, since 2/1 red/white or white/red would allow one of them to instantly figure out the color of his own hat.
Furthermore, they are all red, since they actually raised their hands.
Assuming that all three men are pretty smart, the guy who guessed correctly reasoned the following after waiting awhile:
1. I see that my two partners (A and B) have red hats.
2. If my own hat is white, then A would see B raising his hand, and realize that his own hat is red, since B is raising his hand for seeing A's red hat, and not for my own.
3. Same argument as #2, except switching A and B around.
4. Since neither A nor B were able to deduce the color of their own hate, my own hat, therefore, cannot be white, and is thus red.