There were 3 prisoners in the king's dungeon. two of them had perfect vision but the third prisoner was completely blind.

one day the king gathered the prisoners together and said to them, "i have 5 hats. 2 of the hats are black and 3 are white. i will blindfold each of you and then put one of the hats on each of your heads. i will then put the remaining two hats in another room. next, i will remove your blindfolds. at this point you may look at your fellow prisoners but may not speak to them. the person that can tell me the color of the hat on his head will go free. do not insult me by guessing, however, for if you guess and are wrong, you shall be executed."

the hats were then placed on the prisoners' heads and the blindfolds were removed.

the king then spoke to the first prisoner, a fully sighted man. "what is the color of the hat on your head" the prisoner looked around carefully but finally, not wanting to risk death by guessing, said that he did not know.

the king then spoke to the second prisoner, the other fully sighted man. "what is the color of the hat on your head" this prisoner also looked carefully at the hats upon the heads of the other two prisoners, but finally admitted that he also did not know.

at this point the king turned and began to walk away, assuming that the third prisoner who was blind could not know possibly the answer to the riddle.

but then the blind man spoke. "excuse me, my lord," he said, "but i am certain my hat is white."

surprised, the king turned around. he saw that the blind man was correct and ordered him to be freed at once.

how did the blind prisoner know the color of his hat?