Time is very important to us. When and by whom were the watches or clocks invented？ Perhaps nobody knows. How could we tell time if there were no watches or clocks anywhere in the world？The sun was probably the world's first "clock"，except in the far north，where the Eskimos live. There，it's dark most of the winter, and light most of the summer. But in most of the world, people have used the sun for day. If you don't have a clock that shows time, you still know that when the sun shines, it's day, and when it's dark，it's night.  The sun can not only tell you whether it's day or night but also it's morning，noon，or afternoon. When the sun is almost directly overhead, it's noon. People who live near the sea know time from the tides（潮汐）. In the daytime, for about six hours, the water rises  higher and higher on the beach. The same thing take place again at night. There are two high tides and two low  tides every 24 hours. In some places in the world the wind comes up at about the same time every day or changes direction or stops blowing. In these places the wind can be the clock.