A little morning humor

A biologist, a physicist and a mathematician are standing together. They observe two people walking into a house. A few minutes later, 3 people walked out. The biologist replies 'The humans must have reproduced inside the house'. The physicist replies 'There must have been an error of approximation while estimating the number of people who walked in'. The mathematician replies 'Now, if one more person walks into the house, there will be exactly zero people inside the house'.