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.

Americans love cars. They go everywhere in them. 85% of people in the US go to and form work by car. And most adults have driving licenses. Why does this car culture exist?

How it started

America's love of cars started after the war when soldiers returned home from World War Π to rebuild their lives. They borrowed money from the government to buy houses and cars which became the symbols of status（地位）. The more money they had, the bigger their cars were.

Making of roads

During the war, President Eisenhower noticed what good roads Germany had. He decided to build new four-lane（四车道）roads in America. He said is something happened suddenly, the two-lane roads wouldn't be able to carry all the cars that would suddenly leave the cities. Car and oil companies liked his idea and building started.

Car lovers

Not just teenagers are crazy about cars. Some Americans love their cars so much that they paint their cars beautifully. These are called Art Cars. Every April there is an activity in Houston, Texas, where they show their cars.

Pollution

Cars have polluted the environment. American President Bush refused a worldwide law that is against pollution. Many countries were angry about it. Bush said he had to think of the American economy（经济）and all the American people that make money from cars. Factories say they want to make cars that pollute less. But others say making cars that pollute less will never be as good as having fewer cars.

    Venus is close to the earth．It is the second planet from the sun．The earth is the third planet from the sun．Venus is similar(相似的)to the earth in size and mass (质量)．It is often described as the earth's"sister"．

Scientists have been learning about Venus for a long time．First they looked at Venus through telescopes(望远镜)．But Venus is covered with thick clouds．Scientists could not see Venus well through the clouds．So they made guesses about it．

For a long time，scientists thought that Venus had water and plants．They thought Venus might have animals too．But they didn't know for sure．

Then scientists found a way to learn more about Venus．In'the 1960s，they began sending probes(航天探测器)to Venus．The probes flew through the thick clouds．They took pictures of Venus up close．They found out many things too．The probes sent the pictures and things they found out back to the scientists．

The scientists learned a lot from the probes．First they learned that most of their guesses were not right．And they learned that Venus is very hot．

After that，scientists knew they could not send people to Venus．But they still wanted to know more about it．So they sent new probes．These new probes took great pictures of the planet．The pictures showed that Venus has plains (平原)and mountains!And in some ways，Venus looks like the earth．

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任务一：简要回答下列问题。

    Have you ever had trouble sleeping in a new place?

    Lots of people do. And now researchers from Brown University in Rhode Island think they know why. They found that one-half of the brain "remains more awake" than the other half when people are trying to sleep in a new place.

The sleep findings were reported in Current Biology by Brown University. In their report, the researchers said many people report they have a harder time sleeping the first night at a hotel or other places outside their home. They call it "first-night influence". "In Japan they say, 'If you change your bed, you can't sleep,'" said Yuka Sasaki, one of the report's writers. "You don't sleep very well in a new place. We all know about it."

    The researchers measured brain waves (测量脑波) for 35 volunteers over two nights in a laboratory. The two nights were a week apart (间隔). They found during the first night the left half of the brain was more active than the right half. This was during the first deep-sleep period, the researchers said.

    Sasaki said a lot of questions remain.

    Researchers did not keep measuring brain waves all night long. So, they don't know if the left half keeps "watch" all night, or whether it "works in shifts (轮换) with the right half later in the night". They also do not know why the brain activity, at least during the first period of deep sleep, is always on the left half.

    For some, this research may be calming. It is good to know that our brain is "looking out for us" in a new place. But it may not help with sleep. That brain activity, at least according to this new research, makes it harder to get the sleep people need to wake up well rested in the morning.

    Tigers are the largest wild cats in the world. They usually go out to hunt for food when it is dark. And they hunt alone most of time. When a tiger sees an animal, it moves quickly and then jumps on the animal to kill it. Sometimes it can take the tiger several days to finish eating its food. When the tiger is full, it will cover the dead animal with leaves. Later, when the tiger is hungry again, it comes back to eat some more.

    Tigers are different from others in the cat family. They like water and often jump into rivers to swim. Some tigers live in very cold places-in North Korea, Russia and China. Other tigers live in warmer places-in India and parts of Southeast Asia.

    But now all tigers are in danger. Some people kill tigers to use their skins for clothes, and their bodies for medicine. There is also less land for tigers to live on.