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.
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."