In 1901, H. G. Wells, an English writer, wrote a book describing a trip to the moon. When the explorers (探险者) landed on the moon, they discovered that the moon was full of underground cities. They expressed their surprise to the "moon people" they met. In mm, the "moon people" expressed their surprise. "Why are you travelling to outer space when you don't even use your inner space?" they asked.

    H. G. Wells could only imagine the travel to the moon. In 1969, human beings really did land on the moon. People today know that there are no underground cities on the moon. However, the question that the "moon people" asked is still an interesting one. A growing number of scientists are seriously thinking about it.

    Underground systems (体系) are already in place. Many cities have underground car parks. In some cities, such as Tokyo, Seoul and Montreal, there are large underground shopping areas. The "Chunnel", a tunnel connecting England and France, is now completed.

    But what about underground cities? Japan's Taisei Corporation is designing a network of underground systems, called "Alice Cities". The designers imagine using surface space for public parks and using underground space for flats, offices, shopping and so on. A solar dome (太阳能穹顶) would cover the whole city.Supporters of underground development say that building down rather than building up is a good way to use the earth's space. The surface, they say, can be used for farms, parks, gardens, and wilderness. H. G. Wells's "moon people" would agree. Would you?

    Both seasonal flu, a disease like a very bad cold, and much larger pandemics, like the one caused by H7N9（禽流感） in 2013, are the result of a race between the flu virus  and the immune system which can protect people from illness. Here's how it works.

    Viruses, which cause pandemics, have two important proteins "haemagglutinin(H) and neuraminidase(N).H helps the virus go into a cell .N helps new virus break out of that cell .They are parts of a virus that may be known and dealt with by the immune system

    A flu virus attacks a host cell in order to reproduce itself. But the virus created out of it is quite different now. The virus's genetic material is copied with low fidelity. That difference produces virus which immune system does not always know and deal with immediately. Each year's seasonal virus is therefore a little bit different, and thus requires an updated vaccine to prevent people from getting that disease.

    Pandemics viruses have bigger changes in this process .Many hosts' defences are unprepared for such big changes, which happen three or four times a century .The 1918 pandemics was caused by viruses carrying H1 and N1.Anotber two pandemics in 1957 and 1968 were caused by viruses carrying H2and H3 together with N2.They are numbered by scientists. Much of this change goes on outside humans. A flu is usually from sick binds. Sometimes a bird-flu appears with materiel that enable it to spread among people .The H7N9 is recently worrying scientists in the health center.

    The flu virus sometimes jumps directly from a bird(often a chicken) to a person, but more usually passes through a pig The cells in bird and human are built differently, It means us that the virus needs different sets of material to attack them .Pig cells have features of both so that it can act as a platform in which the virus can fit from one to the other .Even after it has made humans ill, a virus cannot go ca to cause a pandemic unless it also gets the ability to pus easily between people, Luckily, this is something H7N9 has yet to do.

    Have you ever heard someone use the phrase "once in a blue moon"? People use this expression to describe something that they do not do very often. For example, someone might say that he tries to keep away from sweets because they are unhealthy, but will eat chocolate once in a blue moon. Or someone who does not like to. Go to the beach might say, "I visit the shore(岸边) once in a blue moon." While many people use this phrase, not everyone knows the meaning behind it.

    The first thing to know is that the moon itself is never actually blue.  This is just an expression. The phrase "blue moon" actually has to do with the shape of the moon, not the color.

    As the moon travels around the earth, it appears to change shape. We have certain names for certain shapes of the moon. For example, when we can see a small part of the moon, it is called a crescent moon. A crescent is a shape that looks like the tip of a fingernail(指甲). When we can see the moon appear in the sky as a thin crescent, it is called a new moon. When we can see the whole moon, it is called a full moon. Usually, there is only one full moon every month. Sometimes, however, there will be two full moons in one month. When this happens, the second full moon is called a "blue moon".

    It's predicted(预言) that there will only be 15 blue moons over the next 20 years. As you can see, a blue moon is a very unusual event. This fact has led people to use the expression "once in blue moon" to describe other" very unusual events in their lives.

    Some people claim they" never forget a face". But what does that saying mean? Is there really no limit to the number of faces a person can remember? A new study at the University of York in England has found that, on average, people can remember as many as 5,000 faces.

    The study is the first time that scientists have been able to put a number to the abilities of humans to recognize faces. The research team tested people on how many faces they could remember from their personal lives and in the media. They also tested them to see how many famous faces they recognized. Rob Jenkins works in the psychology department at the University of York. He said the researchers' study centered on the number of faces people actually know. He said the researchers were not able to discover whether there is a limit on how many faces the brain can handle.

    In the study, people spent one hour writing down as many faces from their personal lives as possible. At first, they found it easy to come up with many faces. But by the end of the hour, they found it harder to think of new ones. Their change in speed let the researchers estimate when they would have run out of faces completely.

    The results showed that these people knew between 1,000 and 10,000 faces. Jenkins explained that some people may have a natural ability for remembering faces." There are differences in how much attention people pay to faces and how well they process the information," he said. Jenkins also said it could be because of different social environments. Some people may have grown up in more populated places. Therefore, they may have had more social contact throughout their lives.

    The people in the study included 25 men and women between 18 and 61 years old. Researchers think age may be an interesting area for further research. "It would be interesting to see whether there is a peak age for the number of faces we know", Jenkins said. He said it is possible that we gather more faces throughout our lifetime. But, he added, there also may be an age at which we start to find it harder to remember all of those faces.

    The study suggests our facial recognition abilities enable us to deal with the many different faces we see on the screens, as well as those we know, like family and friends. Today, facial recognition technology is used in many ways, including by law enforcement agencies to prevent crime and violence. Governments use it to keep secret areas secure and, in extreme cases, control populations. .Even Facebook uses facial recognition. For example, when you "tag" or name a friend, Facebook technology may recognize the person's face from a different picture you had shared before.