Science becomes engaging when students know that what they learn in the classroom relates to their own lives and communities. This article describes a project in which students use field exploration and online software to design virtual solutions to improve the hydrology of their school yard. The project is called Teaching Environmental Sustainability--Model My Watershed (TES-MMW). In the unit, students collect data to learn about land features that affect where water flows in their school yard. They also use a scientifically valid watershed model to learn more about their watershed and its current health. Using the Model My Watershed software application, students can virtually change land cover and implement conservation practices such as installing a rain garden, porous pavement, or a "green" roof, which involves installing a waterproof membrane over a traditional roof and covering it with a growing medium and vegetation. (A rain garden is a shallow depression near a runoff source that is planted with deep-rooted grasses and native plants.) Armed with data from their school yard investigation and model outputs, students virtually redesign their school yard to improve watershed health. In this way, students use evidence-based reasoning to become engaged citizens.