There are three important parts to this story, the first is that Callisto was very beautiful. This is lucky, considering her name means "most beautiful". The second is that, at some point, she was turned into a bear and/or killed. The third is that it had to do with Zeus and Artemis. See, Callisto was one of Artemis' nymphs (and thus not supposed to be getting down with any men), but Zeus got wise and came to her as Artemis and seduced her. (Hooray for subliminalized lesbianism!) One story says that Zeus immediately turned her into a bear to hide her from Hera, which of course didn't work, since Hera promptly got Artemis to "accidentally" shoot her. Another story says that Zeus just busted and peaced, and that Artemis found out when they were bathing at a spring and she noticed that Callisto was chubbier than she ought to be (she was pregnant - my opinion on this is that they were all bleeding at the same time and she noticed that Callisto was late, but, that's just me) and Artemis then changed her into a bear and hunted her down. Other stories say that Hera hunted her down much later and the death involved Callisto's own son from that union, Arcas. Together, she and her son are the Big Bear and the Little Bear, respectively.