A professor sits with a farmer in a train. Bored, the professor says to the farmer: “I ask you a question, if you can’t answer it, you give me $5; then you ask me a question, if I can’t answer it, I give you $500, what do you think?”

The farmer nods. The professor asks the farmer: “What is the distance between the Earth and the Moon? ”The farmer silently takes out $5 and give it to the professor. The farmer asks the professor: “What animal has three legs when ascending a mountain and four legs when descending a mountain? ”The professor thinks hardly but couldn’t find an answer, so he reluctantly pulls out $500 for the farmer. The farmer takes the $500 and prepares to nap, the professor asks: “What animal is it!? ”The farmer takes out $5 and give it to the professor, then he falls asleep.