TITLE: Mathematical joke regarding three people leaving a house
QUESTION [36 upvotes]: I don't get this mathematical joke - can someone explain?

From Wikipedia:
A physicist, a biologist and a mathematician are sitting in a street café watching people entering and leaving the house on the other side of the street. First they see two people entering the house. Time passes. After a while they notice three people leaving 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.

REPLY [38 votes]: I think the point of the punchline is that the mathematician simply solves the math problem as observed, and appears totally unconcerned with the impossibility of having -1 people in a house in the real world.  
Sciences like physics and biology are about explaining the real world, but in mathematics explaining the real world is not a requirement.