When students are engaged in scientific modeling, they are able to notice patterns and develop and revise representations that become useful models to predict and explain—making their own scientific knowledge stronger, helping them to think critically, and helping them know more about the nature of science. To illustrate, this article describes a unit on evaporation and condensation that incorporates four essential aspects of scientific modeling. In the unit, students construct, use, evaluate, and revise evaporation and condensation models to understand how a solar still cleans dirty water.