I was reading about where "I'll give you a piece of my mind" came from and then got into idioms. Here is a good list. In reading some of those, I can certainly understand why non-English speakers have trouble with understanding them.
What are your favorite idioms? I sometimes get confused if something is an idiom or just a saying, but I think an idiom is something that is understood in the language, but otherwise doesn't make sense, right? Like, when my mom used to say, "beauty is as beauty does," that's a saying. But on that idiom list, "Elvis has left the building" is clearly an idiom. Still, some of the others there seem pretty easy to figure out, so I am no longer so sure.