A biologist, a statistician, a mathematician and a computer scientist are on a photo-safari in Africa.

They drive out into the savannah in their jeep, stop and scour the horizon with their binoculars.

The biologist: "Look! There's a herd of zebras! And there, in the middle: a white zebra! It's fantastic! There are white zebras! We'll be famous!"

The statistician: "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!"