This is quite an old joke, I just added a bit (to knock economists) and retell it here in my own words:
A physicist, an economist, a mathematician, and an engineer, travelling together by train, cross the border between England and Scotland.
The physicist points out of the window and says: "Look, the sheep in Scotland are black!"
The economist doesn't even look up, and says: "According to my models there are no black sheep, so I will ignore it and maybe consider it as special case later."
The mathematician looks at the physicist wearily and says: "Stop generalising. Just say there are black sheep in Scotland."
The engineer looks around in disbelief, sighs deeply, and says: "All we know for certain is that there is at least one sheep in Scotland that is black on at least one side."