讲解文本：
grind one's teeth 磨牙 grind 磨，碾 Some people grind their teeth while they are asleep. 有些人睡觉时磨牙齿。 My roommate Jack always grinds his teeth in his sleep, which is a totally disaster. 我室友Jack喜欢睡觉磨牙，这对我就是个灾难。
疯狂练习吧！
