"Engineer's Game" joke

A Programmer and an Engineer were sitting next to each other on an airplane. The Programmer leans over to the Engineer and asks if he wants to play a fun game. The Engineer just wants to sleep so he politely declines, turns away and tries to sleep.
The Programmer persists and explains that it's a real easy game. He explains,"I ask a question and if you don't know the answer you pay me $5. Then you ask a question and if I don't know the answer I'll pay you $5."
Again the Engineer politely declines and tries to sleep.
The Programmer, now somewhat agitated, says, "O.K., if you don't know the answer you pay me $5 and if I don't know the answer I pay you $50!"
Now, that got the Engineer's attention, so he agrees to the game. The Programmer asks the first question, "What's the distance from the earth to the moon?"
Then Engineer doesn't say a word and just hands the Programmer $5.
Now, its the Engineer's turn. He asks the Programmer, "What goes up a hill with three legs and comes down on four?"
The Programmer looks at him with a puzzled look, takes out his laptop computer, looks through all his references and after about an hour wakes the Engineer and hands the Engineer $50.
The Engineer politely takes the $50 turns away and tries to return to sleep.
The Programmer, a little miffed, asks, "Well what's the answer to the question?"
Without a word, the Engineer reaches into his wallet, hands $5 to the Programmer, turns away and returns to sleep.