完形填空。

When Mary Smith was a 1, she wanted to be a teacher, 2she liked children. When she was twenty, she began 3in a small school. She was a 4teacher, and she laughed a lot with the children in her class. They 5 her very much.

One day, one of the girls in her class asked her, "Miss Smith, 6does my father's 7become white before his beard (胡子) does?" Mary laughed and answered, "I don't know. Who8?" The other children in the class laughed9they heard this. Then a boy said, "I know! A man's hair is 10years older than his beard."