A door-to-door salesman is selling encyclopaedias. He knocks on the door of a house and when the man of the house answers, the salesman begins to expound the virtues of his product. He hasn’t got very far into his sales pitch when the prospective customer says, “It’s no good to me mate, I don’t need it.” The salesman is a bit taken aback, but nonetheless he carries on and begins to detail all the knowledge contained in the volumes he is trying to sell. Again the man of the house interrupts he and says, “Look, I told you I don’t need it; I’ve got a teenage son who knows everything.”