what is a idiom?
Is this one, " don't judge a book by it's cover"
An idiom is a phrase that has more meaning than what the individual words mean.
Yes, your example is an idiom.
http://www.usingenglish.com/reference/idioms/
Is this one, " don't judge a book by it's cover"
Yes, your example is an idiom.
http://www.usingenglish.com/reference/idioms/