An undigested bit of beef...?

Sort of trick trivia question - quickly now, how many ghosts visited Ebenezer Scrooge in Dickens's "A Christmas Carol"? The immediate reaction is to say three: the ghosts of Christmas past, Christmas present and Christmas yet to come. But what you have to remember is that the whole thing started off by Scrooge being visited by the ghost of Jacob Marley, his one-time business partner, so the proper answer is four. Try it on your friends - but don't give them too long to think about it.