A biologist, a physicist and a mathematician meet on the terrace of a café and see a couple enter the hotel across the street. A few minutes later, the couple leaves the hotel with a child.

The biologist comments: it's clear, they have reproduced.

The physicist: no, it's simpler than that: 2 people entered, 3 left, except for the error of experience, nothing happened.

The mathematician: do you realize it now only takes one person to enter the hotel for it to be empty?