Are you sure?

Noah amuses (and confuses) me whenever I ask him this question. I am pretty sure he doesn't understand what it means. Here is how it usually goes.

Me: Noah, do you need to go potty?
Noah: No.
Me: Are you sure?
Noah (meaning yes): No.

And he continues right on out the door.

* For the record (and completely unrelated) - my favorite thing he says lately is "see you tomorrow." It comes out like this: See you monner!