A set of strategy profiles is here said to be closed under rational behavior (curb) of it contains all its best replies. Each curb set contains the support of at least one Nash equilibrium in mixed strategies, but there are perfect Nash equilibria that are not contained in any minimal curb set. It is shown that every game with compact strategy sets and continuous payoff functions possesses at least one minimal curb set, that every minimal curb set is identical with its best replies and that it is contained in the set of rationalizable strategy profiles.