A True McGuffin

'It might be a Scottish name, taken from a story about two men in a train. One man says "What's that package up there in the baggage rack?", and the other answers "Oh, that's a McGuffin". The first one asks "What's a McGuffin?". "Well", the other man says, "It's an apparatus for trapping lions in the Scottish Highlands". The first man says "But there are no lions in the Scottish Highlands", and the other one answers "Well, then that's no McGuffin!". So you see, a McGuffin is nothing at all.'

-- Alfred Hitchcock in an interview with François Truffaut (1966)