All of you have seen big, puffy clouds "floating" in the sky on a summer's day. Why do clouds stay in the sky? Why don't they fall down? Maybe clouds "float" for the same reason that helium balloons float. Maybe they are lighter than air. That can't be true because we know that clouds are made of water. Water is not lighter than air - water does not float. So why don't clouds fall out of the sky? The two biggest reasons that clouds stay in the sky are 1) small drops, and 2) wind. Small drops of water fall more slowly than big drops. The reason is that as drops fall through the air, the air pushes back on them. Because small drops have less mass and more surface area than large drops, they have a harder time pushing the air out of the way.