This easy-to-perform demonstration shows students how the water cycle, specifically the processes of condensation and evaporation, purifies Earth's water supply. Just mix up some water, dirt, and gravel in a glass bowl, place a cup in the middle, cover bowl with plastic wrap, and leave it in the sun for a few hours. When you check back later in the day, the cup will contain clean, clear water. In the upper elementary and middle school grade levels, this demonsteration would make a perfect addition to an earth science unit on the water cycle.