A biologist, a statistician, and a mathematician are sitting at a cafe. Across the street, a man and a woman enter a building; ten minutes later, they emerge with a child.

“They’ve reproduced,” says the biologist.

“No,” says the statistician. “It’s an observational error. On average, 2.5 people went each way.”

“You’re both wrong,” says the mathematician. “The conclusion is obvious. If someone goes in now, the building will be empty.”