A mathematician, a biologist and a physicist are sitting in a street café watching people going in and coming out of the house on the other side of the street.

First they see two people going into the house. Time passes. After a while, they notice three people coming out of the house.

“The measurement wasn’t accurate,” says the physicist.

“No, no,” says the biologist, “they have reproduced.”

“I don’t think so,” says the mathematician. “If exactly one person now enters the house, then it will be empty again.”