Question

expand the theme:you can't judge a book by it's cover

Answer 1

you can't judge someone with his outer appearance

Answer 2

Don’t judge a book by its cover:

This proverb "Don’t judge a book by its cover" refers, we must not decide about something just looking a small part about it. Similarly, this phrase define, we must not judge anything about the cover, We must think before speaking, we must have a good knowledge on the topic we speak before we tell anything about it. This is very useful and most famous proverb.

Example: If you find a boy/girl always playing and he/she doesn't learn if you think it's wrong because he/she may learn well, It is wrong to judge about someone without knowing about their background. Also, you must not judge wrongly without knowing about someone's tallest.

Conclusion: Never judge anything wrongly, without knowing about it clearly.