“Don’t judge a book by it’s cover” is a common and very useful idiom. Imagine you are in a book store and you want to buy a new book. Suppose you pick up a book you have never heard of before. How do you decide if you should buy the book or not?
All you can really do is look at the cover and read the back of the book and “take a guess”. The point is that it’s impossible to know if the book will actually be good or not by just looking at the design on the front of the book. The content or the written material is what is truly important, not the design on the cover (the front of the book). This applies to many areas in life. There are many good websites that look ugly, and there are many bad websites that look quite “pretty”. You can tell someone, “Don’t judge a book by it’s cover” if you are trying to recommend a website that doesn’t look great, but actually is very useful. Sometimes we also do this with people. A girl might not be very attractive physically, but she may be a wonderful person inside. Someone who is extremely beautiful on the outside also may be a horrible and mean person on the inside (ie. A bad and/or selfish personality)
So, if you “Judge a book by it’s cover” it means you judge something’s value or quality by the way it looks or appears, rather than by the quality of the actual content. Now use this. It’s a great phrase!
“Don’t judge a book by it’s cover” is a common and very useful idiom. Imagine you are in a book store and you want to buy a new book. Suppose you pick up a book you have never heard of before. How do you decide if you should buy the book or not?