Leda was the queen of King Tyndarus of Sparta. One night she slept with both her husband and with Zeus in the form of a giant swan.
It kind of makes you wonder -- who got the sloppy seconds? If it was Zeus, well, that's just wrong; Zeus shouldn't have to put up with that. If it was Tyndarus, wasn't Zeus enough for her? Swan or not, you would think that after sex with a god, you'd want to rest a while.
Leda's two sets of twin children were born from two eggs: the Dioscuri, Castor and Pollux, and Helen and Clytemnestra. Castor was the mortal son of King Tyndarus, and Pollux was Zeus's son. Helen and Clytemnestra were born from the second egg. Some believed that Castor and Pollux were both immortal, and that the female twins were the mortal daughters of Tyndarus.