When I read chicken soup for the pre-teen soul , I came across "Never put rocks in your mouth". What does that mean? Can somebody tell me? Thanks a lot!
It's not any sort of standard idiom. Having just read the story, it seems like something a four-year-old would say in a list of "things you shouldn't do." I don't really think she was thinking about it in a figurative sense about harmful words - especially because she also just listed "don't watch movies with kissing it them" in the same breath.

In other words, don't start using this phrase as though it has a common, accepted meaning in English conversations.
I too would have said that it means exactly what it says.

At no time should you ever take any rocks and place them in your mouth. They are probably covered with germs, and they are not edible!

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It means that words can be used to hurt people just like rocks. Carrying a word like a hard rock in your mouth will also hurt you.

There is another popular saying you may have heard in English, "Sticks and stones may break my bones, but words will never hurt me." However, this saying is wrong. Words can and do inflict more pain than the biggest stick or the heaviest stone could ever cause. Cursing, mocking, blaming, insulting…we all know that this simple method of just saying a few well chosen words can manipulate and hurt people in ways we can't even imagine.

It means words do hurt us...always. What we say, how we say it, what we mean by it, does affect us and other people. Words have energy and power. Words can heal or maim; comfort or kill. Never put rocks in your mouth.
Thank you very much Glacierlily!
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Rocks in your mouth are lies. As a rock will destroy your teeth so will lies chip away at the enamel of your soul.