A zoologist, a statistician, and a mathematician are sitting across the street from an empty house.

A zoologist, a statistician, and a mathematician are sitting across the street from an empty house.

While they are sitting there they see two people enter the house. A short while later they see three people leave the house.

The zoologist says "They must have reproduced."
The statistician says "Our initial count must have been wrong."
The mathematician says "If one more person goes into that house it will be empty again."