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: All I know is that they were made of wood. W: A professor at a dental school in California recently presented a paper 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 about a hundred years ago. The ivory had turned a very dark color, and I guess, looks like wood. M: Yes, I've seen some ivory that was dark brown color. W: It seems that ivory turns dark easily with age, especially if it's exposed to drinks, such as coffee, tea, or wine. I guess that's how the story got started. M: What's 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 up- per jaws, the bottom teeth wear down into the shape of a human tooth. (23)