A biologist, a physisist, and a mathematician are standing outside of a building. Two people walk in. After a few minutes, three people walk out.
"Aha!" the biologist says, "they must be breeding!"
"No, no," the physisist replies "this is could be bad. In order to preserve the laws of thermodynamics, someone else must go into the building."
The mathematician replys "But if someone else goes into the building, it will be empty."