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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."

ETA: (Yeah, I'm a math geek.)