I bought my friend an elephant for his room

I bought my friend an elephant for his room...
He said "Thanks!"
I said "Don't mention it."

This is nominally an elephant joke, but it is really a joke about English. To explain the humor, I will first explain the phrase "elephant in the room". The phrase is used when there is something (the elephant) that is obvious to everyone, but that no-one wants to talk about.

The joke, then is that the elephant that he bought for his friend is an idiomatic elephant in the room as well.