It is bad to have food stuck between your teeth for long periods of time. This is because food attracts germs; germs produce acid, and acid hurts your teeth and gums. Flossing (使用牙线) helps to remove the food that gets stuck between your teeth. This explains why flossing helps to keep your mouth healthy, but some doctors say that flossing can also be good for your heart.

    It may seem strange that something you do for your teeth can have any effect on your heart. Doctors have come up with a few ideas about how flossing works to keep your heart healthy. One idea is that the germs that hurt your teeth can leave the mouth and travel into your blood.Germs that get into the blood can then attack your heart. Another idea is based on the fact that when there are too many germs in your mouth, the body tries to fight against there germs. For some reason, the way the body fights these mouth germs may end up weakening the heart overtime.

    Not every doctor agrees about these ideas. Some doctors think that the link between good flossing habits and good heart health is only a coincidence. The incidence (发生率) of two or more events is completely random, as they do not admit of any reliable cause and effect relationship between them. For example, every time I wash my car, it rains. This does not mean that when I wash my car, I somehow change the weather. This is only a coincidence. Similarly, some doctors think that people who have bad flossing habits just happen to also have heart problems, and people who have good flossing habits just happen to have healthy hearts.

    The theory that flossing your teeth helps to keep your heart healthy might not be true. But every doctor agrees that flossing is a great way to keep your teeth healthy. So even if flossing does not help your heart, it is true to help your teeth. This is enough of a reason for everyone to floss their teeth every day.

    Science has a lot of uses. It can uncover laws of nature, cure diseases, make bombs, and help bridges to stand up. Indeed science is so good at what it does that there's always a temptation(诱惑) to drag it into problems where it may not be helpful. David Brooks, author of The Social Animal: The Hidden Sources of Love, Character,and Achievement, appears to be the latest in a long line of writers who have failed to resist the temptation.

    Brooks gained fame for several books. His latest book The Social Animal, however, is more ambitious and serious than his earlier books. It is an attempt to deal with a set of weighty topics. The book focuses on big questions: What has science revealed about human nature? What are the sources of character? And why are some people happy and successful while others aren't?

    To answer these questions, Brooks surveys a wide range of disciplines（学科）. Considering this, you might expect the book to be a dry recitation of facts. But Brooks has structured his book in an unorthodox(非常规的), and perhaps unfortunate, way. Instead of introducing scientific theories, he tells a story, within which he tries to make his points, perhaps in order to keep the reader's attention.So as Harold and Erica, the hero and heroine in his story, live through childhood, we hear about the science of child development and as they begin to date we hear about the theory of sexual attraction. Brooks carries this through to the death of one of his characters.

    On the whole, Brooks' story is acceptable if uninspired. As one would expect, his writing is mostly clear and, to be fair, some chapters stand out above the rest. I enjoyed, for instance, the chapter in which Harold discovers how to think on his own. While Harold and Erica are certainly not strong or memorable characters, the more serious problems with The Social Animal lie elsewhere. These problems partly involve Brooks' attempt to translate his tale into science.

    Wild animals are our friends, but many of them are getting fewer and fewer. We should try to protect them. The four animals below are now in danger.

     Tibetan Antelopes

    Tibetan antelopes are medium - sized animals. They mainly feed on grass. They are usually found in groups of about 20. They are killed for their wool, which is warm, soft and fine and can be made into expensive clothes. Although people can get the wool without killing the animals, people simply kill them before taking the wool. The number of them is dropping year by year. There are less than 75,000 Tibetan antelopes left in the world, down from a million 50 years ago.

    Golden Monkeys

    Golden monkeys are mainly found in Sichuan, Gansu, Shanxi provinces and Shengnongjia mountainous area of Hubei Province. Golden monkeys have golden - orange fur. They move around in the daytime, usually in groups of as many as 100 to 200 heads, or 20 to 30 heads. They feed on fruits and young leaves of bamboos. But people are destroying the environment where they live. Trees and bamboos are disappearing, so golden monkeys have less and less to eat.

    Elephants

    Elephants are very big and strong. They are bigger than any other animals on land. They are grey and have long trunks and tusks. They have poor eyesight, but very good hearing and smell. They can lift heavy things and break down branches with their trunks. Elephants are very friendly towards each other and towards their neighbours. Normally, they live in a group for many years. Young male elephants do not leave the group until they are about 12 years old. Now, there are very few elephants in the world. The number of them is becoming smaller and smaller because their living areas are used for fanning. Also, people hunt them for their tusks.

    Wolves

Wolves are not very big. They have grey fur. Solves have very good eyesight, hearing and smell. Wolves' food is various. They eat animals, insects and snails. They are friendly to each other and never attack people. They do not kill for fun. Wolves are in danger, too. They are losing their living areas because people cut down forests. Soon they will have no home or food.

    We are all busy talking about and using the Internet, but how many of us know the history of the Internet?

    Many people are surprised when they find that the Internet was set up in the 1960s. At that time, computers were large and expensive. Computer networks didn't work well. If one computer in the network broke down, then the whole network stopped. So a new network system had to be set up. It should be good enough to be used by many different computers. If part of the network was not working, information could be sent through another part. In this way computer network system would keep on working all the time.

    At first the Internet was only used by the government, but in the early 1970s, universities, hospitals and banks were allowed to use it too. However, computers were still very expensive and the Internet was difficult to use. By the start of the 1990s, computers became cheaper and easier to use. Scientists had also developed software that made "surfing" the Internet more convenient.

    Today it is easy to get on-line and it is said that millions of people use the Internet every day. Sending e-mail is more and more popular among students.

    The Internet has now become one of the most important parts of people's life.

    “An apple a day keeps the doctor away.” Similarly, if we drink a little more water every day, we may find that we need to go to the doctor a lot less often. A healthy diet is made up of water and a great many different kinds of foods. We know that healthy foods like fruits, vegetables and fish, are important as they provide us with energy and fiber, without giving us too much fat and sugar. But why is water important?

    The human body needs water to survive. Water is important as it carries poisons and waste out of our bodies. However, doctors advise us to drink plenty of water, much more than the small amount we need to stay alive.

    Some doctors believe that water is important for our health in all kinds of ways. They believe that a great many illnesses and health problems start as a result of dehydration(脱水)—a lack of water in the body. Most people are dehydrated at least some of time, although they probably don't know it. It is surprising to learn that if we feel thirsty, we are already in the first stage of dehydration. In fact, we are advised to drink water before we feel thirsty.

    These doctors think that health problems such as headaches, high blood pressure and asthma(哮喘) could be helped if people drink more water. They also believe that drinking more water can help people who are overweight. Water contains no calories so it will not make us put on weight. If we drink water, we will probably drink less sugary soft drinks such as Coca-cola. Doctors also believe that drinking more water may stop people from overeating. They say that many people think that they are hungry when they are, in fact, thirsty. Doctors suggest, therefore, that we eat less food and drink more water. There are still many health problems that cannot be solved by drinking water, so we should not stop going to seeing the doctor completely. However, if we make a decision to drink water every day, we may find that we need to go to the doctor a lot less often.

    There are two main things that make aircraft engineering difficult: the need to make every component as reliable as possible and the need to build everything as light as possible. The fact that an airplane is up in the air and can't stop if anything goes wrong, makes it perhaps a matter of life or death that its performance is completely dependable.

    Given a certain power of engine, and consequently a certain fuel consumption, there is a practical limit to the total weight of aircraft that can be made to fly. Out of that weight as much as possible is wanted for fuel, radio navigational instruments, passenger seats, or freight room, and of course, the passengers or freight themselves. So the structure of the aircraft has to be as small and light as safety and efficiency will allow. The designer must calculate the normal load that each part will bear. This specialist is called the "stress man." He takes notice of any unusual stress that may be put on the part as a precaution against errors in manufacture, accidental damage etc.

    The stress man's calculations go to the designer of the part, and he must make it as strong as the stress man says is necessary. One or two small parts are always tested to prove that they are as strong as the designer intended. Each separate part is tested, then a whole assembly—for example, a complete wing, and finally the whole aeroplane. When a new type of aeroplane is being made, normally only one of the first three made will be flown. Two will be destroyed on the ground in strict tests. The third one will be tested in the air.

    When a plane has passed all the tests it can get a government certificate of airworthiness, without which it is illegal to fly, except for test flying.

    Making the working parts reliable is as difficult as making the structure strong enough. The flying controls, the electronic equipment, the fire precautions, etc. must not only be light in weight, but must work both at high altitudes where the temperature may be below freezing point and in the hot air of an airfield in the tropics.

    To solve all these problems the aircraft industry has a large number of research workers, with elaborate laboratories and test houses, and new materials to give the best strength in relation to weight are constantly being tested.