Mr. Bean v/s Einstein

Einstein & Mr.Bean sitting next to each other on a long flight..

Einstein says, Let's play a game..
I will ask you a question, if you don't know the answer, you pay me only $5 and if I don't know the answer, I will pay you $500..

Einstein asks the first question : What's the distance from the Earth to the Moon..?

Mr.Bean doesn't say a word, reaches his pocket, pulls out a $5..

Now, it's Mr.Bean's turn..
He asks Einstein : What goes up a hill with 3 legs and comes down on 4 legs..?

Einstein searches the net and asks all his smart friends..
After an hour he gives Mr.Bean $500..

Einstein going nuts and asks : Well,so what goes up a hill with three legs and comes down with four..?

Mr.Bean reaches his pocket and gives Einstein 5$ !!!