SageModeler is free, web-based, and open-source software designed to engage students in systems thinking through constructing, using, evaluating, and revising system models to explain phenomena and design solutions. Students can create system models and run simulations, comparing their model output to real-world datasets to answer driving questions. SageModeler does not require installation or login and bandwidth requirements are low. It is available in several langauges. Teachers can easily assign work and students can peer-evaluate models with a simple share link.