Three scientists sit at a bar, overlooking an apartment building. Before the first beer, they notice two persons entering the building. A few hours and beers later, they notice that three persons leave the building. Now they all get agitated, because they all need to explain what has happened. 

The biologist claims that the two have mated, and the third specimen is the result.

The physicist claims that what they all witnessed had a major measurement error, and that in fact there must have been three entering the building earlier.

The mathematician smiles. She knows the right answer, and spells it out: "Don't worry. When the next person enters the building, it will become empty."

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