The Hat Trick:
Three Mathematicians are sitting on chairs in a row. A game master has three white and two black hats in a box. The Mathematicians know that. For each Mathematician the game master draws a hat by random. Now every Mathematician can see the hat's colour of his foremen, but not his own. (Hence the front person can't see any hat!). The Mathematicians are told to receive a BIG prize when one can tell his hat's colour with certainty. In the first place, no one says a word. Suddenly the front person announces correctly the colour of his hat.
How could he know? What is the colour of his hat?