#mathematic #game-theory Cooperate & punish.  Sharks, after eating a lot of fish have dirty teeth, they move to a place in the ocean to have their teeth cleaned by small fish, they could easily close their jaws when the fish is in it but if they do so, the others small fishes won't be likely to cooperate with this shark in the future. Small fishes can also harm the shark by biting its mouth, in which case it is likely that the shark will punish it. An optimal strategy, a [[John Nash]] equilibrium can be reached by cooperating most of the time and punish when the other defect. On the long-term, both are better off cooperating Bat also plays tit for tat, sharing blood