英语笑话~A difficult question

A poet and a scientist were traveling together on a plane. The scientist was bored and said to the poet, ”Hey, you, do you want to play a game? I’ll ask you a question, and if you get it wrong, you give me $5.Then,you ask me a question, and if I can’t answer it, I’ll give you $5.” The poet thought about this for a moment, but he decided against it, seeing that the scientist was obviously a very bright man. He politely turned down the scientist’s offer. The scientist, who was really bored, tried again.” Look, I’ll ask you a question, and if you can’t answer it, you give me $5.Then you ask me a question, and if I can’t answer it, I’ll give you $50.” The poet agreed ”Okay,” the scientist said,” What is the exact distance between the Earth and the Moon?” The poet, obviously not knowing the answer, didn’t stop to think about the scientist’s question. He took a $5 bill out of his pocket and handed it to the scientist. The scientist happily accepted the bill and at once said, “Okay, now it’s your turn.” The poet thought about this for a few minutes, then asked, “All right, what goes up a mountain on three legs, but comes down on four?” The bright glow quickly disappeared from the scientist’s face. He thought about this for a long time taking out his notebook and making numerous calculations．He finally gave up and took out his laptop, using his Multimedia Encyclopedia．As the plane was landing the scientist gave up. He handed the poet a $50 bill. The poet accepted it happily, getting ready to stand up, ”Wait!” the scientist shouted, “you can’t do this to me! What’s the answer?” The poet looked at the scientist and calmly put a $5 bill into his hand. 文章大意： 一位诗人和一位科学家乘同一架飞机旅行。科学家很无聊，于是对诗人说:“嘿，你，你想玩个游戏吗?我会问你一个问题，如果你回答错了，你就给我5美元。然后，你问我一个问题，如果我回答不上来，我就给你5美元。” 诗人考虑了一下，但他决定拒绝科学家的请求，因为这位科学家明显是一个非常聪明的人。所以他礼貌地拒绝了那位科学家的提议。 那个很无聊的科学家不甘心，又问了一次。“你听我说，我问你一个问题，如果你回答不了，你就给我5美元。然后你问我一个问题，如果我答不上来，我就给你50美元。” 诗人说:“好吧。”接着科学家就问，“地球和月球之间的确切距离是多少?” 显然诗人不知道答案，但是却还在思考科学家提出的问题。最后他从口袋里掏出一张5美元的钞票递给了科学家。 科学家开心地接过这5美元，马上说:“好了，现在轮到你了。” 诗人沉思了几分钟，然后问道:“仔细听好了，什么东西上山时是三只脚，但是下山时却是四只脚呢?” 科学家脸上的笑容逐渐消失了。他思考了很长一段时间，接着拿出笔记本，在上面做了大量的计算。最后他放弃了计算，拿出平板电脑，翻看着多媒体上的百科全书。当飞机着陆时科学家放弃了。最后他递给诗人一张50美元的钞票。 诗人高兴地接过了，准备站起来，“等等!”科学家大喊道:“你不能这样对我!所以答案是什么？” 诗人看着科学家，平静地将一张5美元的钞票塞进他的手里。