idiom
don't judge a book by its cover
見た目だけで誰かや何かを判断すべきではない
Meaning
you shouldn't form an opinion on someone or something based purely on what you see on the outside
Example
She may look serious, but **don't judge a book by its cover**.
彼女は真剣に見えるかもしれませんが、**見た目で判断してはいけません**。