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 Biologist’s conclusion: “They have reproduced”.
The Mathematician: “If now exactly 1 person enters the house then it
will be empty again.”