Zeus was not a one woman kinda god. Pretty much if it had a skirt on he would sleep with it (too bad too: almost everyone wore togas at that time). Not long after his affair with his cousin Metis, Zeus found out that she was pregnant with his child. He went to the oracle of Delphi to get the stats on the baby. What he found did not amuse him: The child Metis carried within her now was a girl, and she would be a powerful goddess and a great ally to Zeus. Her next child would be a boy, and when he grew up would be more powerful than Zeus, and would take over the god's kingdom. Zeus, like most ancient gods, didn't think this was so cool, so he swallowed Metis to prevent her from having any children. This didn't stop the baby from wanting to be born, however. About nine months after he slept with his cuz', he got a really pounding headache. It was so bad, that he asked Hephaestus (one of his other children) to crack open his skull. The Hephster did this, and out popped Athena in full battle armor and everything. Since Metis didn't actually give birth to the child and because she was in his head and couldn't have any more children, Zeus got to keep his throne and his dignity, and everyone lived semi-happily on Mt. Olympus. Oh, by the way, Metis still lives in Zeus' skull, and she gives him good advise (she was the first goddess of wisdom) when he actually decides to listen to the voice in his head.