> A biologist, a physicist and a mathematician were sitting in a street cafe watching the crowd. Across the street they saw a man and a woman entering a building. Ten minutes they reappeared together with a third person.

> - They have multiplied, said the biologist.

> - Oh no, an error in measurement, the physicist sighed.

> - If exactly one person enters the building now, it will be empty again, the mathematician concluded.

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