听力原文：W: Hi, Mike! You missed a great history lecture this morning. Where were you? M: Oh, I overslept again. This really is becoming a bad habit. What did Professor Brown talk about today? W: Can you believe it? We talked about George Washington's false teeth. M: Oh, all I know it is said they were made of wood. W: Well, that turns out as a myth. A professor at a dental school in California presented a paper three months ago showing that Washington's teeth were made of many things, including, can you imagine, elephant ivory and cow's teeth, but not wood. M: But why do people say Washington's teeth were made of wood? W: A set of his teeth that were made of elephant ivory were shown publicly at various exhibitions around the country about a hundred years ago. The ivory had turned a very dark color, and I guess, look like wood. M: Mm, yes. I've seen some ivory that was dark brown color. What about cow's teeth? W: You know cows have teeth just on their bottom jaws. After about five years of biting grass off against their hard upper jaws, the bottom teeth wear down into the shape of a human tooth. So they, too ,were used to make false teeth fifty years ago. M: That's really interesting. Well, this will certainly teach me not to oversleep again. I wonder whether little-known Professor Brown will have false teeth next week. It is said that Washington's teeth were made of