Zebras

A biologist, a statistician, a mathematician and a computer scientist 
are on a photo-safari in Africa. As they're driving along the savannah 
in their jeep, they stop and scout the horizon with their binoculars.

The biologist: "Look! A herd of zebras! And there's a white zebra! 
Fantastic! We'll be famous!"

The statistician: "Hey, calm down, it's not significant. We only know 
there's one white zebra."

The mathematician: "Actually, we only know there exists a zebra, which 
is white on one side."

The computer scientist: "Oh no! A special case!"