A Back Versus a Forward

A rugby back and a rugby forward were sitting next to each other on an airplane. The forward leans over to the back and asks if he wants to play a fun game. The back just wants to sleep so he politely declines, turns away and tries to sleep. The forward 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 back politely declines and tries to sleep.
The forward, 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 back's attention, so he agrees to the game. The forward asks the first question, 'What's the distance from the earth to the moon?'. The back doesn't say a word and just hands the forward $5.
Now, its the back's turn. He asks the forward, 'What goes up a hill with three legs and comes down on four?' The forward looks at him with a puzzled look, takes out his laptop computer, looks through all his references and after about an hour wakes the back and hands the back $50. The back politely takes the $50 turns away and tries to return to sleep.
The forward, a little miffed, asks, 'Well what's the answer to the question?'
Without a word, the back reaches into his wallet, hands $5 to the forward, turns away and returns to sleep.
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