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!"