What?....Read Carefully and Then Re-Think

Sitting in a cafe, a mathematician, a physicist, and a biologist watch people going in and coming out of a house across the street.

First, they see two people go into the house. Then, after a considerable time passes, three people come out of the house.

The physicist concludes: "The measurement was not accurate."

The biologist concludes: "They must have reproduced."

The mathematician concludes: "If one person now enters the house, it will be empty again."