An engineer, a physicist, and a mathematician were on a train heading north, and had just crossed the border into Scotland.
•The engineer looked out of the window and said "Look! Scottish sheep are black!"
•The physicist said, "No, no. Some Scottish sheep are black."
•The mathematician looked irritated. "There is at least one field, containing at least one sheep, of which at least one side is black."
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variants:
•The statistician : "It's not significant. We only know there's one black sheep"
•The computer scientist : "Oh, no! A special case!"
Well I liked it, anyway. And the Maths Jokes picture above!