When Hephaestus was born and his mother, Hera, saw how ugly he was, she threw him off of Olympus, where he landed at the edge of the sea, breaking his legs.
Eventually, a wave carried him to the bottom of the sea where he was found by Thetis, a naiad, who thought he was a tadpole. Once Thetis realized he was a baby, she decided to keep him.
One day, Thetis saw Hera at a festival, where Hera saw a necklace Thetis was wearing that Hephaestus made her. After hearing the story of where she got it, she explained to Thetis that he was her son and he came back to Olympus, where he married Aphorodite in exchange for making Hera things.