I say "what?" after everything someone says, even if I heard them the first time.
Also I often stroke over the screen of my notebook ... I don't know why, I just randomly do it, but only with the backside of my forefinger, not with any other fingers. I don't do this when I'm sitting in front of my PC, only with the notebook. When I started stroking once I need to stroke along the whole screen because I begin to feel bad otherwise.