Complex systems thinking is increasingly important for scientific literacy. However, high schools teachers are not prepared to teach it, and most attempts to improve students’ complex systems thinking have been unsuccessful. This investigation examines the outcomes of a participatory agent-based simulation, the UVA Bay Game, for 33 AP biology students. Their pre- and post-treatment essays are examined within a Structure, Behavior, Function framework to probe whether students showed more expert thinking about the complex system of the Chesapeake Bay watershed. Preliminary results show that students’ post-treatment writings showed more expert understanding through increased focus on the behavior and function of system component. Implications for the use of participatory simulations as a vehicle for teaching complex systems are discussed.