So, there was this mathematician, physist, and biologist who went into this building and counted to make sure it was empty, because it was set to be demolished.
So they finish up and count and there is nobody inside. They go across the street and wait. After some time, two people enter. A few more minutes pass and three people exit.
The physist says "We must had miscounted".
The bioligist says "They must have reproduced when they were in there".
The mathematician says "Allright, when one more person enters the building will be empty.