Albert Einstein gets on a plane and sits down next to an elementary kid. They strike up a conversation and the kid starts bragging about how smart he is, as kids will do. Einstein chuckles and decides to teach the kid a little lesson. "Let's have a contest. I'll ask you a question, and if you don't know the answer, you give me five dollars. Then you can ask me a question, and if I don't know the answer, I'll give you five hundred dollars." The kid thinks this is a pretty good deal, and offers Einstein the first question.
"How many miles away is the moon?" Einstein asks.
The kid thinks for a moment and says, "I really don't know," then hands Einstein five dollars. "My turn," says the kid. "What goes up a hill with three legs, and comes down with four?"
Einstein ponders this long and hard, thinking about all his scientific knowledge and his extensive reading on philosophy and world folklore, but he can't come up with anything. Finally, he gives in. "Alright, you got me." He gives the kid five hundred dollars, then he asks, "Alright, so tell me: What goes up a hill with three legs, and comes down with four?"
The kid shrugs and hands Einstein five dollars.