In some metaphysical mythology, Nemesis produced the egg from which hatched two sets of twins: Helen of Troy and Clytemnestra, and the Dioscuri, Castor and Pollux. While many myths indicate Zeus and Leda to be the parents of Helen of Troy, the author of the compilation of myth called Bibliotheke notes the possibility of Nemesis being the mother of Helen. Nemesis, to avoid Zeus, turns into a goose, but he turns into a swan and mates with her anyway. Nemesis in her bird form lays an egg that is discovered in the marshes by a shepherd, who passes the egg to Leda. It is in this way that Leda comes to be the mother of Helen of Troy, as she kept the egg in a chest until it hatched.