A professor and a farmer make a bet.
The professor says, "If I ask a question and you don't know it, I'll give you five dollars.
You ask a question that I don't know, and I'll give you five hundred, okay?
The farmer agrees.
Professor: How far is the moon from the earth?
The farmer hands the professor five yuan.
Farmer asks: What kind of animal has three legs up the mountain and four legs down the mountain?
The professor thinks hard and has no choice but to lose five hundred.
The farmer took the money and the professor asked: What kind of animal is it?
The farmer handed the professor five yuan, said I do not know, and then went to sleep.