An astronomer, biologist, an engineer and a mathematician were crossing the
border into Scotland from England on a train when they saw a
field with a black sheep in it.

The astronomer said, "Look -- all sheep on Earth are black."

The biologist said, "Look, in
Scotland the sheep are black".

The engineer replied, "No, in
Scotland some of the sheep are black".

The mathematician rolled
his eyes to heaven and said, very patiently, "In Scotland, there
exists at least one field, in which there is at least one sheep
which is black on at least one side".

Cheers,
Kirt