A mathematician, biologist and 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.
After a while they notice three persons coming out of the house.
The physicist says: “The measurement wasn't accurate.”
The biologist concludes: “They have reproduced.”
The mathematician says: “Now if another person enters the house, it'll be empty again.”