A mathematician, physicist and biologist watch a house

A physicist, a biologist and a mathematician are sitting in a street cafe watching people entering and
leaving a house from the other side of the street.

They see two people entering the house.

After a while they see three people leaving the house.

The physicist says, "Our measurement wasn't accurate."

The biologist says, "They reproduced!"

The mathematician says, "If one more person enters that house, it will be empty."