A Physicist, a Mathmatician and a Biologist are sitting in a café, looking out the window at a deralict house across the street.  After a few minutes two people walk into the house.  A few minutes pass, then they see three people leave the house.

The Biologist says that the people must have reproduced.

The Physicist says that the original measurement must have contained an inaccuracy.

The Mathmatician says that now, if one more person enters the house it will be empty again.