Ian Ball teaches the third lecture of MIT's 14.12 Economic Applications of Game Theory, working through the Prisoner's Dilemma as the entry point into formal strategic reasoning. He builds up the idea of a dominant strategy, one that beats every alternative regardless of what an opponent does, and contrasts it with best responses that depend on a guess about the other player's move. The session works through how to eliminate strategies that no rational player would ever choose, narrowing down which options are actually worth analyzing in a competitive game. Ball uses payoff matrices and worked examples on the board to show students how to spot dominance relationships quickly and why they matter for predicting outcomes before more complex equilibrium concepts get introduced later in the course. Seventy-six minutes of standard blackboard lecture aimed at students building the toolkit for economic game theory.