The House Conundrum

A mathematician, a biologist, and a physicist are watching as two people enter a house. Some time later, three people can be seen leaving the house.

The scientists make three different observations.

The physicist: "Our measurements were flawed."

The biologist: "They must have reproduced."

The mathematician: "If one person now enters the house, it will be empty."
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Note: That is a well-known math joke. Here's another famous one:

Wanted - Schrodinger's Cat, dead and alive.