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Yep.

When you tell a child. "Be nice, don't be mean" that is generally a positive statement.

However, if you say "How nice" with a hint or hefty helping of sarcasm in your tone it has a very not pleasant meaning.

It's like saying "How stupid" :)

Yeh in that context 'be nice' means 'be kind'.

We actually can stop using words but just communicate through tones, faces, gestures.
LOL

Body language gives a word the meaning, not the other way around. Maybe I should stop reading and writing at all ;-)