Two men were sitting together in a plane. They were on a long journey. One of the men was a businessman. 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 businessman asked. “We can ask each other riddles,” the farmer said. “OK. Let’s make rules first,” the businessman said. “If you don’t know the answer to a riddle, you pay me $ 100. And if I don’t know the answer, I’ll pay you $ 100.” “That’s not fair ( 公平) .You are a businessman with much knowledge. You know more things than I do. I am just a farmer,” the farmer said. “That’s true,” the businessman said. “What do you want us to do?” “If you don’t know the answer to a riddle, you pay me $ 100. And if I don’t know the answer, I’ll pay you $ 50,” the farmer said. The businessman thought about this, then he said, “OK. That’s fair. Who will go first?” “I will,” the farmer said.“Here is my riddle. What has three legs when it walks, but only two legs when it flies?” The businessman thought for a long time and said, “Mm, that’s a good one. I’m afraid I don’t know the answer.” He gave the farmer $ 100, then he said, “Tell me the answer.” “I don’t know,” the farmer said. Then he gave the businessman $ 50.