Two strangers were sitting together in a plane. They were on a long journey. One of the men was a teacher. The other was a farmer.

  They sat without talking for a while，then the farmer said，“Let’s do something to pass the time. ”

 “What do you want to do?” the teacher asked.

 “We can ask each other riddles (迷语)，”the farmer said，“You start. ”

 “Let’s make the rules first，”the teacher said，“And to make the game more interesting. Let's play it for money. If we don’t know the answer to a riddle，we have to pay a dollar. ” The farmer thought about this for a while，then he said，“That’s not fair，you are a teacher，an educated man. You know more things than I do. I am just a farmer. ”

 “That’s true，” the teacher said，“What do you think we should do?” The farmer said，“If you don’t know the answer to a riddle，you pay me 100 dollars. If I don’t know the answer to a riddle，I'11 pay you 50 dollars. ”

  The teacher thought for a while，and then he said，“OK. That’s fair. Who'11 go first?”

 “I will，” the farmer said.“Here’s my riddle. What has three legs when it walks，but only two legs when it flies?” The teacher thought and thought，“What has three legs when it walks，but only two when it flies?” Then he said，“That’s a good one. I don’t know the answer. ”He gave the farmer 100 dollars and said，“Tell me the answer. What is it?”

 “I don’t know，either. ” The farmer said，and gave him 50 dollars.