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.

The physicist: "The measurement wasn't accurate."
The biologists: "They have reproduced".
The mathematician: "If now exactly one person enters the house then it will be empty again."