Hi this is probably very easy for you guys, but I can't figure this out. Heated air rises right? Then why is it that when I put a bowl of soup in the microwave and heat it in a plactic Tupperware-like container, then remove and cover it -- does a vacuum like effect occur? It would seem to me tha the hot, steamy air would want to push the cover off. Instead it sucks the cover down. Why?