Ian Ball teaches this session of MIT's 14.12 Economic Applications of Game Theory, working through the concept of Nash equilibrium, the state in which every player is playing a best response to everyone else so no one benefits from switching strategy alone. He builds the idea through concrete examples, including two friends deciding between attending a Celtics game or a Red Sox game and a hide-and-seek matchup, using both to show how equilibrium concepts apply to everyday coordination and conflict problems. The lecture also covers mixed strategies, explaining why players sometimes need to randomize their choices to keep opponents from predicting their behavior. Running just over an hour, the session is blackboard-based and builds formal definitions step by step from the examples rather than starting with abstract notation.