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Mirrors have no meaning other than the meaning we assign to them, so a mirror will lie as much as its user lets it. For example, the when I look in the mirror I see an ugliness that others don't. So, haven't I caused the mirror to "lie?"

even you just admit that the guilty is the user not the mirror, so... mirror still not lies

Sure, but you've missed the point. The user can easily cause the mirror to lie to the user. No inanimate object can lie. Objects are meaningless other than the meaning we assign them.

The point being, you cannot change a person by telling them the "mirror never lies."