2．Natural ways of keeping time
In ancient times，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．
Man-made things
The sand clock is made of two glass balls joined by a narrow neck．The top ball was filled with sand．The sand slowly moved through the neck into the bottom ball．People knew how much time had passed when all the sand had fallen to the bottom ball．
People also made the water clock．They made a small hole near the bottom of a pot．Then they filled the pot with water．Markings inside the pot showed how much time passed as the water dripped （滴） out of the hole．
Nature's Clocks in Living Things
◆Animals'Body Clock
Animals do not need clocks to know time．They have a way of telling time by their bodies．Birds know when to fly to warmer places before winter．Some animals know when to keep more food for the cold winter months．Some fish know when it is time to move up the river and lay eggs．
◆Plants have their own clocks
Plants also have their own clocks to keep time．Plants know when to open flowers or when to drop their leaves．
◆The Body Clock in Human Beings
People also have their own body clocks．When we get used to our lives，our body clocks can be very accurate （準(zhǔn)確的）．It can tell us when it is time to wake up．It can also tell us when to eat or to go to sleep．
38．How could an ancient farmer know when to go back home in the daytime？C
A．By looking at the moon．
B．By looking at the stars．
C．By looking at the sun．
D．By feeling whether tired or not．
39．According to the passage，how can animals know time？A
A．They use body clocks．
B．They ask other animals．
C．They use the sun and the moon．
D．They make something to tell time．
40．In what way is the sand clock the same as the water clock in telling time？D
A．Both of them have a neck．
B．Both of them have two glass balls．
C．Both of them can be used only once．
D．Both of them give the time by quantity．
41．What is the best title for the whole passage？C
A．Man-made clocks
B．Nature's clocks
C．Time and clock
D．How people keep time．
