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（北极）is tilted（倾斜）toward the sun. In those months the Northern Hemisphere（半球）gets more hours of sunlight than the Southern Hemisphere（半球）. 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.

    Smog(雾霾) is a big problem in many Chinese cities. But it seems that besides wearing a mask, few of us know what has caused the smog and what to do to beat it. Chai Jing, a former news hostess with China Central Television, discussed the questions in her documentary Under the Dome.

    The 103 - minute film was published on major Chinese video - sharing websites on Feb 28. In the film, Chai visits polluted places and talks to officials and scientists. She comes to the conclusion that burning too much coal and oil are the main causes of PM2.5 and smog.

    To make things worse, according to Chai, the coal and oil we are burning is of poor quality and don't meet the standards for environmental protection. Therefore they have produced more harmful gas to pollute the air.

    To solve the problem, Chai suggests that we should clean our coal and oil or replace them with cleaner energy such as natural gas. In her film, Chai also pointed out that ordinary people can do something to help control smog too. For example, we live a greener life by using more public traffic. Or if we see things that could harm the environment, we can try to stop it. In one scene from the film, Chai sees a dirt mound in a building site exposed in the air, and asks the person in charge to cover it. When she sees a restaurant cooking without any suitable filter facilities, she calls the hotline 12369. The restaurant installed the filter a week later.

    Chen Jining, the new minister of environmental protection, praised Chai. "This film encourages ordinary people to care about the environment. This is what we need in the future. "

    Scientists in Britain and the United States have made a new kind of enzyme(酵素) that eats plastics (塑料). With its help, plastics can be turned into smaller parts and go away quickly. Scientists believe that, in the future, it could solve the problem of plastic pollution.

    The enzyme is able to break down PET, a form of plastic. PET was first made in the 1940s and is now widely used to make plastic bottles. It can remain in the environment for many years and pollute large areas of land and water. So it has always been a headache to deal with the waste from PET and other plastics.

    The researchers made the discovery (发现) while examining the structure of a natural enzyme in Japan. They found that the natural enzyme was helping to break down PET plastics. So, they decided to make small changes to the enzyme by adding some amino acids (氨基酸). It made the natural enzyme's plastic-eating abilities work faster and better.

    John McGeehan, one of the lead researchers, said, "We've made an improved enzyme. It's better than the natural one. And we are trying to make more improvements to it." He went on, "The enzyme is not harmful to humans or animals, and it is friendly to the environment. So we are considering putting it into wider use."

    The team is now trying to make the enzyme break down PET in large quantities (大量地).

    "We'll see that the plastic pollution may be stopped with this technology. However, there is still a long way to go," John McGeehan added.

    The American Computer &. Robotics Museum was open to visitors in 1990 in Bozeman, Montana. At first it was planned to build in Princeton, New Jersey. The founders moved to Bozeman in 1988 and after looking at the area they decided to build the museum there. With Yellowstone National Park to the south and Glacier National Park to the northwest, the museum attracts thousands of visitors from all 50 states and over 50 countries. The American Computer &. Robotics Museum has been written about in many publications such as The New York Times, The New Yorker, Los Angeles Times, QST and USA Today.

    When visitors first enter the museum, they are shown a video introducing the museum and its collections. All visitors are given the chance to join in tours led by friendly tour guides. We have had visitors aged from3 to 93- it is a big family. Over 1,000 exhibitions are shown in the 6, 000-squaremetre museum. Travelling through our museum, you will see the exciting 20,000 years of history of the information age. Technology becomes easier to accept when one learns about its origins(起源) in the natural process(进程) of human development.

    Many people have the wrong ideas about pigs. Pigs are actually(买际上) very clean animals. On farms, they live in dirty places, so they become very dirty. But in the wild, pigs keep themselves very clean.

    They are also very smart. They may he smarter than clogs. So pigs can learn things from people.

    Pigs are very friendly animals. Some people raise (饲养) them as pets. Of course, people raise the smart kind, not the big kind. Small pigs are very cute. And they don't break things in the house. Big pigs usually live outside on the farms, not in people's houses.