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judge a book by its cover, one can't

judge a book by its cover, one can't idiom

One can't rely on outward appearances to know what something or someone is really like. For example, He seems very quiet, but you can't judge a book by its cover. [First half of 1900s]

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.