The physicist, the biologist, the mathematician

Seen on the net.

A mathematician, a biologist and a physicist are sitting in a street cafe. They notice a dark house across the street, and they decide it must be empty.

Then they see two people pull up in a car and go into the house. After awhile they notice three people coming out of the house.

The physicist: "Actual measurement proved our theory incorrect."
The biologist: "They must have reproduced."
The mathematician: "If now exactly one person enters the house then it will be empty again.