参考答案

                                   MYSTERY OF TIME
    If you can read a clock, you can know the time of day. But no one knows what time itself is. We cannot see it. We cannot touch it. We cannot hear it. We know it only by the way we mark its passing. For all our success in measuring the tiniest parts of time, time remains one of the great mysteries of the universe.
    One way of thinking about time is to imagine a world without time. Then there could be no movement, because time and movement cannot be separated. A world without time could exist only as long as there were no changes. For time and change are linked. When something changes, you know time has passed. In the real world, changes never stop. Some changes happen only once in a while, like an eclipse of the moon. Others happen repeatedly, like the rising and setting of the sun. People have always noted natural events that repeat themselves. When people began to count such events, they began to measure time.