Ha ha I was thinking the same thing before between idioms and metaphors and asked about it with my English teacher. The main thing to remember is that, correct me if I'm wrong with this, when you are using a metaphor, you are comparing two things. And I believe you do it without using the words "like" or "as". I'm gonna think of some examples, right now I really can't think of any good ones. As for idioms, it is how it's been defined. An idiom or idiom phrase is something that means something different than the words used for it. Maybe like the phrase "you are an eyesore" or something like that. You get the idea. I'll think of a good example for metaphors next time. Hope this helps though.