This is an old one that I have hacked about to make it fit, so I hope it still works..
There were a business user, a B.A. and a developer on a road trip in the UK when they crossed the border into Wales. (This was antevirum, so that kind of behaviour was allowed back then).
They saw a sheep on a mountainside.
The business user cried out "Look! All the sheep in Wales are black!"
The B.A. tutted and said "Actually, all we can say is that there is at least one sheep in Wales and it is black down one side."
The developer woke up from nursing his hangover in the back seat, peered out of the window and said "How do you know its a sheep?"