A physicist, a biologist, and a mathematician are sitting in a café and notice people going into and coming out of the house across the street.
First they see two people going into the house.
Time passes.
After a while, they notice three persons coming out of the house.
The physicist: “The measurement was not accurate.” The biologist: “They have reproduced.” The mathematician: “If one person enters the house, then it will be empty again.”
