An engineer, a biologist, and a mathematician stand outside of an empty house and watch as two people enter. A little while later, three people walk out. The engineer says, "It appears our assumption was wrong, there was one person in the house before." The biologist says, "It appears that the two people must have reproduced somehow, and that explains the third person." They both look toward the mathematician for his explanation. "It's simple, isn't it," the mathematician responds, "If one of them goes back into the house, it'll be empty again!"