Meaning:

You use this proverb to say that you can't tell what something/someone is really like just by looking at it. It might be different from what it looks.

If someone says, "You can't judge a book by its cover," he/she advises you not to judge someone or something only by its appearance.

People also say, "You can't tell a book by its cover."

Example:

A: I want to buy this car. It looks great.
B: Have a test drive with someone who knows cars well before you decide. You can't judge a book by its cover.

I know you want to think he's honest and trust him, but be careful. You can't tell a book by its cover.

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