Three logicians, Albert, Bill and Carl, are wearing hats, which they know are either black or white but not all white. Albert and Bill can each see the other two logicians, but not Carl, who is facing away and cannot see them.
Each is asked in turn if they know the color of their own hat. The answers are:
What color is Carl's hat and how does he know?
(Taken from http://einstein.et.tudelft.nl/~arlet/puzzles/logic.html)