Empty house
A biologist, a physicist, and a mathematician were having lunch in a restaurant. They see, across the street, two people walk into a house. One of them comments that the house has two people in it. After a few moments, three people walk out of the house. The physicist says, "Our initial observation was wrong." The biologist says "They must have reproduced." The mathematician is silent for a moment, then remarks, "If exactly one more person goes into the house, it will be empty again."
(Source: http://forums.customerssuck.com/archive/index.php/f-19.html)
(Source: http://forums.customerssuck.com/archive/index.php/f-19.html)
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