A biologist, a physicist and a mathematician were sitting in a street café watching the crowd. Across the street, they saw a man and a woman entering a building. Ten minutes later, they reappeared, with a third person.
“They have multiplied,” said the biologist.
“Oh no, it’s an error in measurement,” the physicist replied.
“If exactly one person enters the building now, it will be empty again,” was the mathematician’s conclusion.