When deciding whether to get on the freeway during rush hour, did you ever stop to consider that if you did so, you would slow everybody ELSE down? When we ask our students that question, they typically laugh--their only consideration is how long their own trip will take. When a decision maker does not account for all the costs or benefits associated with a decision, there exists what economists call an "externality." Externalities are commonplace, and can cause significant problems (or create significant benefits) for firms and society. This article describes an online simulation game that introduces students to the concept of externalities, and helps them understand and develop ways of at least partially neutralizing negative externalities and expanding positive externalities. The game is based on a common situation with which most if not all students will have familiarity, that of traffic congestion. The article introduces the game, presents the rationale behind using a simulation game for teaching the concept of externality, and discusses how the game contributes to analytical thinking, multiple framing, reflective exploration of meaning, and practical reasoning.