Cheryl's favorite joke:
A mathematician, a physicist and a biologist are sitting at a coffee shop and they're watching a house across the street. Two people walk into the house and then three people come out.
The biologist says, "Oh, my God, they've reproduced!" The physicist says, "No, the measurement must have been wrong." And the mathematician says, "I don't know about that, but if another person goes into that house, it'll be empty again."
Explanation: The mathematician, seeing two people go into a house and
three coming out, reasons that the population inside the house is minus one. So one more going in makes it zero - the house is empty.