A physicist, a biologist and a mathematician are sitting on a bench

A physicist, a biologist and a mathematician are sitting on a bench, watching a house on the other side of the street. Initially they decide that the house must be empty. Then 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.”

This joke is an occupation joke. Each of the people in an occupation joke tend to say something that identifies them with the faults associated with their occupation, or at least something that shows them to think about their occupation to the exclusion of all else.

The physicist is played as the straight man in this joke (which is usually either the person that sets up the joke, or the person that is seen as the normal counter to the weirdness of the other characters), when he admits they must have been wrong.

The biologist refuses to believe that they were wrong, and offers up a biological explanation (an absurd one).

The mathematician doesn't even seem to recognize the inconsistency between what they said and reality, since math allows for negative numbers in most cases.