You go to the dentist. The dentist notices that your teeth are worn down and smooth on the edges. She tells you:
You seem to be grinding your teeth at night.
When you see evidence that a person is doing something, but you're still not 100% sure, say "You seem to be ___ing":
He seems to be sleeping.
The clouds seem to be clearing up, so that's good.
When you "grind" something, it means you rub two hard things against each other until they start to rub parts of each other off. Things that people often "grind" include:
wheat or corn (to make flour or corn meal)
the gears on a car (when someone doesn't know how to drive a stick shift well)
The adjective (and past tense) form of "grind" is "ground", so you will often hear people talk about powders such as:
Use "at night" instead of "in the night" for things that happen again and again during the night.
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