A physicist, a biologist, and a mathematician see two people walk into an empty building. Five minutes later, 4 people walk out. The physicist says "Obviously

A physicist, a biologist, and a mathematician see two people walk into an empty building. Five minutes later, 4 people walk out. The physicist says "Obviously our initial observation was wrong." The biologist says "They must have multiplied while they were in the building!" The mathematician says "However you look at it, if two more people walk into that building it will be empty again."