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.