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 (爱斯基摩人) 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 a clock. Even today， if you don't have a clock， you still know that when the sun shines， it's day， and when it's dark， it's night. The sun can also tell you if it's morning， noon， or afternoon.
People near the sea can tell time from the tides(潮汐). In the daytime， for about six hours， the water rises higher and higher on the beach. And then it goes down and down for about six hours. The same thing happens again at night. There are two high tides and two low tides every 24 hours.
Seamen on a ship learn how to tell time by looking at the moon and the stars. The whole sky is their clock. 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.
A sand(沙) clock is an even better clock. If you had fine dry sand in a glass shaped like the one in the picture， you would have what is called an hourglass. The sand in the hourglass goes from the top part to the bottom(底部) part in exactly one hour. When people turn over the hourglass， the sand will take another hour to go back again.
