A physicist, a biologist and a mathematician are sitting on a bench, watching people entering and leaving the house on the other side of the street. First they see two people enter the house; A while later, they watch three people leave the house.
The physicist says, “The initial measurement wasn’t accurate.”
The biologist counters, “They must have reproduced.”
Finally, the mathematician suggests, “If one more person enters the house, then it will be empty again.”