'nersh: my reply is, invariably, "nothing. nothing at all. i'm male. we can do that. for HOURS at a time.".
i stole that from a comedian named wanda sykes. (sp?)
I reply usually with what I am thinking, unless that'd be something that'd get me into trouble.
In those cases I just reply: "Stuff", and only if the person asks for the third time, will I say what I was thinking. Usually at that point they think that I just got pissed at them and made it up.
I usually give some bs answer, especially if it's going to get me in trouble. I'll say something about the first thing I'm looking at after I come back into focus from my thoughts. "Oh, well I'm working on this program and I'm trying to think of what to do next..."