 The games we’ve modeled to this point have all involved players choosing from amongst pure strategies, in which each seeks a single optimal course of action at each node that constitutes a best reply to the actions of others. Often, however, a player’s utility is optimized through use of a mixed strategy, in which she flips a weighted coin amongst several possible actions. (We will see later that there is an alternative interpretation of mixing, not involving randomization at a particular information set; but we will start here from the coin-flipping interpretation and then build on it in  Section 3.1.)  Mixing is called for whenever no pure strategy maximizes the player’s utility against all opponent strategies. Our river-crossing game from  Section 1  exemplifies this. As we saw, the puzzle in that game consists in the fact that if the fugitive’s reasoning selects a particular bridge as optimal, his pursuer must be assumed to be able to duplicate that reasoning. The fugitive can escape only if his pursuer cannot reliably predict which bridge he’ll use. Symmetry of logical reasoning power on the part of the two players ensures that the fugitive can surprise the pursuer only if it is possible for him to surprise himself.