A Mathematician, a Biologist and a Physicist are sitting in a street cafe 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 persons coming out of the house, and they give their professional opinions of their observations.
The Physicist: "The initial measurement wasn't accurate."
The Biologist's conclusion: "They have reproduced."
The Mathematician: "If now exactly 1 person enters the house, it will be empty again."