Here’s an old (but good) one.
Three people of equal intelligence are standing one behind another all facing forward.
A person nearby has five hats, three red and two white. The three people close their eyes while the other person randomly puts a hat on each of them. When they open their eyes they can not see what color hat they are wearing but the person in back can see what color hat the other two are wearing and the person in the middle can see what color hat the person in front is wearing. The person in front can not see what color hat anyone is wearing. The three people are unable to see the two remaining hats.
When the person in back was asked what color hat he was wearing he looked at the two people in front of him and said, “I don’t know”. When the middle person was asked what color hat he was wearing he looked at the person in front of him and said, “I don’t know”. Finally the person in front said, “I know what color hat I’m wearing”.
What color hat was he wearing and how did he know?