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。

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