Oddly, I read it in a children's anthology. It's a very short, simple story--a boy loves a girl who wears a (color) ribbon around her neck. He can't stop bugging her about why she never takes it off. She tells him he'll find out on her deathbed. They marry, get old, yada yada, she says, "Okay, you can take it off now." He unties the ribbon, and her head falls off.