Honk If You're Horny
Zeus, the king of the Greek gods, was forever having affairs with various women, mortal and immortal alike. In order to conceal these liaisons from his jealous wife, Hera, he would often assume some type of disguise (e.g. a bull, a golden cloud, the exact double of the woman's husband). One of these amours was Leda, the wife of the Spartan king Tyndareos. In order to bed Leda, Zeus assumed the shape of a large, white swan. The fact that this stratagem worked at all says something about Leda's secret predilections.
Walking through a meadow by herself, Leda discovered that she was being watched by swan so gleaming white it almost hurt to look at it. Being intrigued, she approached the creature. He seemed to have no fear of humans and even let Leda stroke his feathers. The swan seemed just as curious about her, nuzzling her arms and somehow unpinning her peplos. Leda found that she thoroughly enjoyed the tickling, tingling sensation of burying her face in the bird's downy feathers. It was at that point that she discovered exactly how friendly the swan was. This was more than Leda had bargained for, but the so far the sensations were pleasant enough so she decided to give herself over to this strange new experience.
That evening, Tyndareos returned home completely unaware of the strange tryst that had occurred a few hours earlier. After eating a meal fit for a king (which he was), Tyndareos' mind turned to thoughts of other entertainments. Since television hadn't been invented yet, he decided upon an evening of sex with his wife. (She seemed to have a contented glow that Tyndareos found very attractive.) Despite the bits of down in Leda's hair and a few marks on her thighs in the shape of webbed feet, Tyndareos had no suspicion of his feathery predecessor.
In fact, the first clue that Tyndareos had that something fowl was afoot was his wife's pregnancy. It started out normally enough, but about halfway through the normal course of events, Leda laid a pair of eggs. This sort of thing will raise the suspicions of even the most thickheadedly trusting of husbands, so knowing the jig was up, Leda disclosed to Tyndareos the whole story of her "goosing". Now, having your wife or daughter seduced by a god was not uncommon in ancient Hellas, and was often looked upon as a sign of divine favor. Taking this revelation in stride, Tyndareos decided to raise whatever hatched from the eggs as his own.
In due time the eggs hatched. Inside each of the eggs was a boy and a girl, making four children in all. One of the eggs contained Kastor and Klytæmnestra, the fully mortal children of Tyndareos. The other held Polydeukes and Helen, the semi-divine children of Zeus.