Three professors, a mathematician, a physicist, and a biologist, were assigned to be the safety committee for the destruction of an old campus building. They watched as workers went in and out to prepare the building, set the dynamite, etc., so they knew they needed a plan to ensure everyone was out before they pushed the button to blow it up.
The three professors agreed that they should lock all the doors except one, and keep count of everyone going in and out of the building. After hours on the second to last day, they scoured the building to ensure no one was left inside, and the next morning watched as the workers went in and out.
When the time came to blow up the building, two workers went in to set the fuses, and when they came out, the safety committee said, everyone would be out and it would be safe to blow up the building. Except that once the fuses were set, THREE workers came out!
The physicist says, "We must have miscounted!"
The biologist says, "They must have reproduced when they were in there!"
And the mathematician says, "Okay, when one more person enters the building, it will be empty!"