In Genesis 38, God puts Onan to death for spilling his seed rather than impregnating his dead brother's widow. Although most parents and Sunday School teachers skip over this story, one woman has chosen to adapt it for kids:

One day, there was a man named Judah who had a young son named Onan and an older son named Er who was married to a woman named Tamar. Because Er was a bad person, God killed him. Seeing that Er had died without having any children, Judah went to Onan and said, "Onan, go to your brother's wife, put your penis in her, and make babies because your brother didn't." Onan knew that any child he had with Judah would be Er's child and not his, and he didn't like that. So when he put his penis in Judah, he didn't put the spermies in her but spilled them all over the ground so that no babies would grow in Judah's tummy.

This made God so mad that he killed Onan, too..

This version is borrowed from the All True Bible Stories website, which appears to be serious. Read the story in context here. There seems to be a typo about where Onan was actually putting his penis...but that's what's in the story.

COMMENTARY
The Onan story is only one highlight of an interlude in the "Joseph" story feauring Judah's hot daughter-in-law Tamar. Tamar also goes on to seduce and trick Judah.

Aside from the possible over-reaction on the part of Jahweh, it is relatively clear that Onan is being punished for failing to impregnate Tamar in accordance with a what is called levirate law. This ordinance requires a brother of a deceased man to marry his widow and get her in the family way. Therefore, it wasn't necessarily the act of seed spilling in and of itself that caused Jahweh's ire, but the fact that Onan was denying seed to Tamar. These may be of comfort to millions of Catholic boys who have attributed their poor eyesite to the deadly sin of monkey spanking (a common warning from the priesthood). Another way to say this is: the Church has invented another sexuality sin and backed it up with ambiguous scripture. Save the little penis for the local priest to wack...don't try it on your own.

However, this was probably of scant comfort to the soul of Onan down in Sheol (how would like your name to be synonymous with masturbation). The fact is, the levirate law was given to Moses in the book of Deuteronomy 25, during the Exodus approximately four hundred thirty years after the Joseph era. So Onan was offed by God for a law that had not yet been written.