This is my all time favourite puzzle; i still remember it after a few years, though i cannot remember where i first read it.
Three men were sentence to death. The king, however, decided to give them a chance. They were told that on their foreheads would be painted a spot of paint, which would be either blue or yellow. They will be able to see the paint on the other two, but not on themselves. The first person to determine the colour of the paint on his own forehead AND give a satisfactory answer would go free.
The king put a spot of yellow paint on all three foreheads. After a while, one of them confidently said his paint was yellow, and explained why. He was set free.
What was his explanation?
(Note: this is a logic question, not a 'trick' one)
Edit: sorry, left out one part: the king told them there was at least one yellow dot among the three of them.