Since kids at this age are so damn persistant and you can't simply drop it without them bugging the shit out of you for the next twenty hours, parents fabricated this "Stork" story in order to divert the truth about the birds and the bees for another ten years.
Mom: Well Billy, one day, a stork brought you to us, and we were very happy.
Child: So, I came from a stork?
Mom: That's right. Now go bug your father.