Dry air has an atomic weight of about 29. If moist air weren't lighter, it wouldn't have any buoyancy, rise, and form clouds. The answer is dry air. You shrink down in your seat and peek out the corner of your eye. Your girlfriend is convulsing with laughter.
Giggles seem to be floating around the lecture hall.
Your only consolation is that Rodney Thomas has tape on his glasses, has a pet ferret, and spends every Saturday night at the Chess club with some weirdo named Ruy Lopez. Inc. You've just decided that your Solaris servers, installed by your predecessor, need to be replaced by Windows NT, and all the Macs have to go. It's the obvious answer. Same old you. Same old thinking.