A king wishes to find a husband for his daughter, so he sets a difficult
exam for all the young men in his kingdom. Three men score top marks. In
order to decide which of the three is most suitable, he sets an
additional test.

He sits all three in a circle and blindfolds them. He tells them that he
will place either a red hat or white hat on each of their heads, and
instructs them to raise their hand if they see a red hat on either of
the others' heads after he has removed their blindfolds. He places a red
hat on each of their heads. "The first that can tell me the colour of
their own hat will marry my daughter." He takes off their blindfolds,
and they all raise their hands. They sit there for a whole hour before
finally one of them gets up and says "I have a red hat." How did he know?