An engineer, a physicist, and a mathematician were traveling on a train from London to Edinburgh. As they entered Scotland they looked out the window and saw a black sheep.
"Look at that, all the sheep in Scotland are black," said the engineer.
"No," replied the physicist. "What you mean is that at least one of the sheep in Scotland is black."
"Not quite correct," added the mathematician. "What you really mean is that at least one of the sheep in Scotland is black on at least one side."