Well, there's nothing like stating the obvious, is there, Bobert? I met a guy once who responded in this fashion...

Him: Hi, whatcha doin'?

You: I'm on my way downtown.

Him: Oh, are you?

You: Yeah, gotta get some groceries...

Him: Oh, do you?

You: Yeah...gotta eat, y'know.

Him: Oh, you gotta eat, eh?

You: (clenching teeth) Yeah. Well, I guess I'll head out...

Him: Oh, will you?

And so on...