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".
I posted this before, but since someone said that they didn't get my explanation, I thought that I would post about it again.
To say "don't mention it" is a way of saying "your welcome" to someone which downplays the importance of the thing that you are being thanked for.
An "elephant in the room" is a phrase meaning something that you don't talk about even if it is obvious. Since the critical thing about an elephant in a room is that you don't mention them, it is humorous that the elephant that the jokester bought his friend is not just an elephant for his room, but also a figurative "elephant in the room" because it is a pretty obvious thing, and the jokester doesn't want to talk about it.