We introduce the idea of a sub game perfect Nash Equilibrium analyzing the entry deterrence game as a simultaneous game to show how there might be several Nash Equilibria but only one sub game perfect one...(ie one that sustains repeated questioning about rational behaviour in every possible sub game of the original game - eben sub games where you don't eve expect to be) . Looked at this way there turn out to be several Nash Equilibria...not all of which are "reasonable". The latter part of the class introduces a simple card game as we begin a new section on games of imperfect information.