An engineer, a physicist, and a mathematician are on a train from London to Edinburg. It will be the first time any of them have been to Scotland.

In Scotland the train passes a field and there is a single sheep standing in that field. The sheep is black.

The engineer says, "Look! The sheep in Scotland are black!".

The physicist sighs, shakes his head, and says, "No...at least one sheep in Scotland is black".

The mathematician sighs, shakes his head, and rolls his eyes, and says, "No...at least one sheep in Scotland is black on at least one side at least some of the time".