[简答题]All languages change over a period of time, for reasons which are imperfectly understood. Or rather, since speech is really a form of human activity, like dancing or playing the piano—and not an entity in itself—it is more exact to say that each successive generation behaves linguistically in a slightly different manner from its predecessors. In his teens the young man is impatient of what he considers to be the unduly stilted vocabulary and pronunciation of his elders and he likes to show how up to date he is by the use of the latest slang, but as the years go by some of his slang becomes standard usage and in any case he slowly grows less receptive to linguistic novelties, so that by the time he reaches his forties he will probably be lamenting the slipshod speech of the younger generation, quite unaware that some of the expressions used in church and law-court were frowned upon by his own parents.