Four men are lined up facing the same direction. One is behind a barrier and can't see anything in front of him. They can't look around. They know that there are four of them and in what position they are standing. They know that there are two black hats and two white hats, but they don't know who's wearing which hats (except for those they see in front of them). They are challenged to figure out which color hat is on their own head.
Once one of them figures it out, beyond a doubt, he will shout out that he knows an answer.
This challenge isn't really fair, though; it's only possible for one man to figure out which color hat is on his head. Which one is he, and how does he know?
*If you know, don't tell. Please.
**Credit for this riddle (as far as I know) goes to Kelly Vincent.