A biologist, a physicist and a mathematician travel together by train. Right after entering Scotland they see a black sheep standing on top of a hill.

“Look at that,” says the biologist. “Apparently the sheep in Scotland are black.”

“Nonsense!” says the physicist. “All we can say for sure is that there is one sheep in Scotland that’s black.”

“I am afraid you are both wrong,” the mathematician explains. “The only thing that is certain is that there is a sheep in Scotland, which is black on at least one side.”