Your girlfriend's mother is staying with you for the week. She and your girlfriend have the same build and the same hair, and from behind you cannot tell them apart. This afternoon you walk into the living room and see a woman stretching on a yoga mat with her back to you. You believe it is your girlfriend. You lean down and kiss the side of her neck. She turns around. She is not your girlfriend. She is your girlfriend's mother. Your girlfriend is upstairs.