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.