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.

This activity quantifies an experience many students have had visiting a beach, or even playing in a sandbox. A summer afternoon walk, barefoot, across sand can burn one's feet. Often people take a few steps and then bury their feet in the cooler subsurface sand as they make their way across the beach. After sunset, the surface sand cools rapidly and buried feet are warmed by the deeper sand that has not cooled off yet. Stopping to rest on an empty fire ring, the beach walker notices how warm the concrete ring is long after the Sun has gone down.