so you're going at it with some girl, banging her from behind. you take her over to the windowsill so she looks out on the street below while you're railing her doggystyle. then, really quietly, your buddy who's been hiding in the closet comes over and you pull a super-quick switcheroo. the girl won't even know you've been replaced. next you sneak out of the room, go outside, and wave up at your girl. "who's that?"
last night my identical twin came over unexpectedly and when he waved up at us from the street my girlfriend thought i was pulling a who's that.