I think this one's pretty good. I'm tired, so it took me a couple times to get it:

A mathematician, a biologist and a physicist are sitting in a street café watching people going in and coming out of the house on the other side of the street. First they see two people going into the house. Time passes. After a while they notice three persons coming out of the house. The physicist says, "The measurement wasn't accurate." The biologist says, "They must have reproduced." The mathematician says, "If one more person enters the house then it will be empty."