① In ancient time, people had to use the sun and the moon to tell time. They got up when the sun came up and worked in the fields until the sun went down.
② The sand clock is made of two glass balls joined by a narrow neck. The top ball was filled with sand. I he sand slowly moved through the neck into the bottom ball. People knew the time by watching how much sand fell to the bottom ball. People also made the water clock. They made a small hole near the bottom of a pot. When the water in the pot dripped(滴) out, marking(刻度) inside the pot showed how much time passed.
③ Animals don't need clocks to know time. They have their body clocks. Birds know when to fly to warmer places before winter. Some animals know when to keep more food for the winter. Some fish know when it's time to move up the river and lay eggs.
④ Plants have their own clocks to keep time. Plants know when to open flowers or when to drop their leaves.
⑤ People also have their own body clocks. When we get used to our lives, our body clocks can be very exact. It can tell us when it's time to wake up. It can also tell us when to eat or go to sleep.
