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.

As I understand it an idiom is a phrase where the meaning cannot be guessed from the constituent parts, like "kick the bucket".

Yes, that's what I thought, too. However, when I read some of the ones on the list, they seemed to make sense, such as "the ball is in your court."

However, when I thought about it, maybe I am just used to them. Surely if someone doesn't know basketball and is just coming to an English-speaking country, they wouldn't get it.

Now, this is a perfect one. One could never figure it out without knowing.