This was published on the groaners listserv.

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 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.”