don't judge a book by its cover 的字面意思是“不要根据一本书的封面来判断它里面的内容”。人们多用这个说法来强调在对人和事物做评价时“不要以貌取人，不能光看表面”，只凭外表不能看出一个人或一件事情的真实情况。

【例句】
We all know that we should never judge a book by its cover, but in reality people do.
我们都知道不应该以貌取人，但在现实生活中人们还是会这么做。

Taotao doesn't look very intelligent, but then you can’t judge a book by its cover.
陶陶看着不是很聪明，但是我们不能以貌取人。