A biologist, a physicist and a mathematician were hanging out on the street (real tough mob)

A biologist, a physicist and a mathematician were hanging out on the street (real tough mob)

when they saw two people get inside a house. After a while they saw three people come out of that same house. The biologist claimed "they have reproduced!", to what the physicist responded "no, I guess our initial measurement had an error, the two people we saw get inside were plus minus one". And the mathematician said "I don't see what the problem is, when someone else gets inside that house, it will be empty again".