Mmm hmmmm.......

"Mmm hmmm...." is great. A good portion of the time, it means the person saying it isn't listening. Kids are the most awesome at it. They LOOK like they're paying attention and are nodding their heads and smiling and acknowledging you, but the reality is, they had checked out of the conversation some time back.
I think I'd be less annoyed if they were to come right out and SAY they weren't listening... :s