So I understand why heated molecules move faster--they have more energy. But, for my example, I'll use a water molecule. A water molecule has a certain density, and that density is more than that of air (which is why water stays in a pot). But when the water is heated to 100 deg. Celsius, it becomes steam. This steam always travels in an upward direction. I'm assuming this is because those H2O molecules have more energy--but why do they always go UP? Why don't they just go in some arbitrary directions? And sorry, I'm not well-versed in Chemistry at all. Actually quite an idiot on the subject. :-( But thanks for any useful input!