WHAT GOES UP...

An American lawyer and a Nigerian are sitting next to each other on a long flight. The lawyer believes that Nigerians are so dumb that he could put something over on them easily, So the lawyer asks if the Nigerian would like to play a fun game.
The Nigerian is tired and just wants to take a nap, so he politely declines and tries to catch a few winks.
The American lawyer persists, and says that the game is a lot of fun. "I ask you a question, and if you don't know the answer, you pay me only $5; you ask me one, and if I don't know the answer, I will pay you $500," he says. This catches the Nigerian's attention and to keep the lawyer quiet, he agrees to play the game. The lawyer asks the first question. "What's the distance from The Earth to the Moon?" The Nigerian doesn't say a word, reaches in his pocket pulls out a $5 bill, and hands it to the lawyer.
Now, it's the Nigerian's turn. He asks the lawyer, "What goes up a hill with three legs, and comes down with four?"
The lawyer uses his laptop and searches all references he could find on the Net. He sends e-mails to all the smart friends he knows, all to no avail. After one hour of searching he finally gives up. He wakes up the Nigerian and hands him $500.
The Nigerian pockets the $500 and goes right back to sleep. The lawyer is going nuts not knowing the answer.
He wakes the Nigerian up and asks, "Well, so what goes up a hill with three legs and comes down with four?...