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Phrase(s): Don’t judge a book by its cover. [and] You can’t tell a book by its cover.
Prov. Do not draw a conclusion about someone or something just from outward appearances. • Just because Sam dresses sloppy doesn’t mean he’s a bad person. Don’t judge a book by its cover. • Jill: How can you be so sure this will be a boring movie? Jane: The poster for it is so boring. Jill: Don’t judge a book by its cover!
McGrawhill's American Idioms And Phrasal Verbs