huh???

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A Mathematician, a Biologist and a Physicist are sitting in a street cafe
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: "The measurement wasn't accurate."

The Biologist: "They have reproduced".

The Mathematician: "If now exactly one person enters the house then it will
be empty again."