A Joke

A physicist, a biologist, and a mathematician were observing an empty house. As they watched, two people entered the house. Shortly after, three people left the house.

The physicist said, "Aha! Our initial assumption was invalid."

The biologist said, "No, I believe they simple multiplied."

The mathematician said, "You know, if one more person enters that house it will be empty again."

(passed on by a co-worker).