Three friends are having lunch in a cafe when... ...a couple walks into an abandoned building across the street, and they come out with a child. The first friend, a biologist, notes, ""They've reproduced."" The second friend, a statistician, remarks, ""No, don't you get it? There's 2.5 people going each way."" The third friend, a mathematician, corrects, ""You're both silly. It's blindingly simple! If someone walks in now, the building will be empty.""