EINSTEIN VS MR BEAN

Einstein and Mr Bean are sitting next to each other on a long flight.
Einstein says lets 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 Earth to the Moon ?
Mr Bean doesn't say a word, reaches in 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 asks :
Well, so what goes up a hill with three legs and comes down with four ?
Mr Bean reaches in his pocket, and gives Einstein $5. =))