This lesson introduces students to complex systems and to basic concepts from the field of system dynamics that lie at the heart of systems thinking. These concepts include stocks and flows, feedback loops, unintended consequences, and the basic principle that the behavior of complex systems can best be understood by looking at the system as a whole, and specifically by analyzing the system’s underlying structure. The lesson introduces these topics through an immersion in (and a role-play simulation of) the dynamics of urban recycling systems, many of which have been thrown into crisis in the past two years. Through this current-affairs example of complex systems in crisis, we identify some key structural features that help to explain how these systems behave over time. We also discover how well-intentioned action can cause negative unintended consequences when we try to intervene in a complex system without understanding how it operates.