A biologist, a statistician, and a mathematician are sitting outside a cafe on a sunny morning, enjoying a cappucino, and geneally watching the world go by. Across the road from them is a large building in clear view. A man and a woman walk along the pavement, and enter the building through an open door. A short while later, a man, a woman, and a child emerge from the building. The three coffee-goers look at each other in bewilderment.
“It is simple”, says the bioligist, “I can explain what we have just seen. The man and the woman have obviously reproduced whilst in the building”.
“No”, says the statistician, “what we have seen is just an example of observational error – on average there were two-and-a-half people both entering and leaving the building”.
“Nonsense”, says the mathematician. “The situation as it stands does not require explanation for it is very simple. The bottom line is that if one more person goes inside, then the building wil be empty”.