During the past several decades, there has been a growing awareness of the ways humans affect Earth systems. As global problems emerge, educating the next generation of citizens to be able to make informed choices related to future outcomes is increasingly important. The challenge for educators is figuring out how to prepare students to think about complex systems and sustainability. This article describes a set of design principles used to create online curriculum modules related to Earth's systems and sustainability. The modules include interactive, computer-based, dynamic Earth systems models that enable students to track changes that occur over time. Embedded prompts help students focus on stocks and flows within the system. This approach helps students to identify important resources in the models (stock prompt), to explain the processes that are changing the availability of the stock (process prompt), and to explore real-world examples (application prompts). We report how students learn about the sustainability of soil, a critical resource for growing food, in the module called "Can we feed the growing population?" We give an example of a model-based task, which shows how students identify stocks and flows associated with the model and how they consider human actions in the system. We discuss educational and research implications of using stocks and flows as a framework to structure students' exploration of dynamic models of Earth systems in teaching sustainability.