What do you know exactly

An American and a Nigerian are sitting next to each other on a long flight.
The American is thinking that Nigerians are so dumb that he can fool them easily. . .
So he 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 persists and says that the game is 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.
This catches the Nigerian's attention and to keep the American quiet, he agrees to play
the game. The American 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 five-dollar bill, and hands it to the American.
Now, it's the Nigerian's turn. He asks the American, 'What goes up a hill with three legs, and comes down with four?'
The American uses his laptop, searches all the references he knows. He uses the air-phone; he searches the Net and even the Library of Congress.
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 goes right back to sleep. The American 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?'
The Nigerian reaches in his pocket, hands the American $5 and goes back to sleep.