下面粗体的英文成语中都会有一个词涉及人体的某个部位。先读一读下面的小故事，然后猜猜由它们组成的成语的意思吧。

Last year my English class was full of characters. That's a polite way of saying it had some unusual people that I'll never forget. One student was such a hard worker that he learned all the idioms in our book by heart. He was always saying things like "I'm on cloud nine", or "I'm green with envy". We never knew if he meant what he said or if he was just practicing English. Another student had a sweet tooth. She would bake lots of bread and cakes and bring them to every class for us to share during breaks. Two students met in the class and fell head over heels in love. We were all invited to their wedding and had a great time. Then there was a student who was always pulling someone's leg. For example, one day before class, he put a long homework assignment on the board and made us think that the teacher had given it. We all had long faces until the teacher came in. Then we realized that someone had played a joke on us.

现在看看这些成语在例句中的用法，你能猜出它们的含义吗？

By heart: I know all my friends' telephone numbers by heart. I never have to look in the telephone book.

have a sweet tooth: I know you have a sweet tooth, so I bought you a box of chocolates.

head over heels in love: Pam and Tony are head over heels in love. They're planning to get married.

pull someone's leg: Tomorrow is not a holiday. Don't believe Rich. He's just pulling your leg.

a long face: Because Judy didn't get an invitation to the party, she's walked around with a long face since yesterday.

成语含义揭晓：

by heart：熟记，背诵

have a sweet tooth：爱吃甜食

head over heels in love：深爱着某人

pull someone's leg：<口>愚弄某人，开某人的玩笑

a long face：郁闷（忧虑）的表情，愁眉苦脸