The word“day” has two meanings. When we talk about the number of days in a year,  we are using “day” to mean 24 hours. But when we talk about day and night,  we are using“day” to mean the time between sunrise and sunset. Since the earth looks like a ball,  the sun can shine on only half of it at a time. Always one half of the earth is having day and the other half night. A place is moved from day into night and from night into day over and over by the spinning（旋转）of the earth. At the equator（赤道）day and night are sometimes the same length（长度）. They are each twelve hours long. The sun rises at 6 o'clock in the morning and sets at 6 o'clock in the evening. For six months the North Pole is tilted（倾斜）toward the sun. In those months the Northern Hemisphere（半球）gets more hours of sunlight than the Southern Hemisphere. Days are longer than nights. South of the equator nights are longer than days. For the other six months the North Pole is tilted away from the sun. Then the Southern Hemisphere gets more sunlight. Days are longer than nights. North of the equator nights are longer than days. Winter is the season of long nights. Summer is the season of long days.

    “Cool” is a word with many meanings. Its old meaning is used to express a temperature that is a little bit cold. As the world has changed, the word has had many different meanings. “Cool” can be used to express feelings of interest in almost everything.

    When you see a famous car in the street, maybe you will say. “It's cool.” You May think, “He's so cool,” when you see your favorite football player.

    We all maximize(扩大)the meaning of “cool”. You can use it instead of many words, such as “new” or “surprising”. Here's an interesting story we can use to show the way the word is used. A teacher asked her student to write about the waterfall(瀑布) they had visited. On the student's paper was just the one sentence, “It's so cool.” Maybe he thought it was the best way to show what he saw and felt.

    But the story also shows a scarcity(缺乏)of words. Without “cool”, some people have no words to show the same meaning. So it is quite important to keep some credibility(可信度).Can you think of many other words that make your life as colorful as the word “cool”? I can. And I think they are also very cool.