Long long ago, people lived a nomadic (游牧民族的) life. When the sun came up, people began to collect food. When the sun went down, they went to sleep.

    All of that changed when they began to build towns. They needed a way to tell time and the first useful time-telling tool was invented─the sundial (日晷). As the sun travelled across the sky, it made shadows (影子) on the face of the time of day.

    The hourglass (沙漏) was another time-telling tool. It was possible to get a general idea of the time.

    In the 11th century, mechanical (机械的) clocks were invented. But they weren't very accurate. They lost fifteen minutes each day and they became more accurate when small springs (小弹簧) were added to move the hands of the clock. As a result of this invention, clocks became smaller.

    Over the next 300 years, many improvements were made to clocks and soon many people could afford a clock or watch. By the 1950s, digital (数字的) watches had been invented in 1967, the atomic (原子的) clock was developed. The atomic clock is accurate to within one second every twenty million years.