[William Porter is wheel-tapping a train]
Managing director's wife: You may think me a little stupid but why do they tap them?
William Porter: Well, you see madam, it's like this. If I tap the wheel with this hammer and the wheel goes clang, then I know that the wheel's there.
Managing director: But supposing that the wheel doesn't go clang?
William Porter: Well, then I know that the train's gone!


--Will Hay (as ) in Oh, Mr Porter!