Good puzzle this:
There are three prisoners in jail, one of whom is blind. The jailer decides to offer the prisoners their freedom if they can solve a puzzle. From a selection of three white hats and two red hats the jailer places one hat on each prisoner's head such that no-one can see what colour hat they are wearing, but the sighted ones can see the colour of the other prisoners' hats.
Freedom is offered if any prisoner can say, with absolute certainty, what colour hat they are wearing.
The jailer asks the first sighted prisoner, and he says that he doesn't know.
He then asks the second sighted prisoner, and gets the same reply.
He finally asks the blind prisoner. At this point the blind prisoner says that he does know what colour hat he is wearing. He tells the jailer the answer, thereby securing freedom for all three.
The question is this:
What colour hat was he wearing, and how did he know?