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定义法是用来说明的一种方法。用定义法展开段落就是用简洁而明确的语言解释限定被说明对象的本质特征，把使读者感到陌生的专业术语或容易引起误解的事情解释清楚或区别开来。请看下面五个例子。

The word hero can be confusing, for it has several meanings. It is often applied to ordinary people who happen to perform an act of great courage—a fireman who saves someone from a burning house at the risk of his own life, for example. Then, the principal character of a play, a novel, or a film is known as the hero of the story, even if he is not particularly brave. But the heroes and heroines that we are going to consider now constitute a third group. They are the giants, the out of the ordinary figures whose superiority fills our hearts with admiration and awe; the men and women who give us a high example to follow, a purpose in life, or sometimes just a dream, because they present the person that we would like to be.

例1对“英雄”这一概念进行了说明限定。作者把英雄分为三类，一类是日常生活中见义勇为的人；另一类为影视等文艺作品中的主人公；再一类是我们心目中要追求、仰慕的英雄。

A hobby can be almost anything a person likes to do in his spare time. Hobbyists build model ships, watch birds, climb mountains and raise flowers. Hobbyists also draw pictures and perform on musical instruments. They collect everything from stamps to coins. People take up hobbies because these activities contain enjoyment, friendship, knowledge and relaxation. Hobbies help people relax after periods of hard work, and also offer interesting activities for persons who are too old to work. Anyone, rich or poor, old or young, sick or healthy, can follow a satisfying hobby.

The work people do is called economic activity. Most people work to earn a living and produce goods and services. Goods are either agricultural (like maize)or manufactured (like cars). Services are such things as education, medicine, and commerce. Some people provide goods; some provide services. Other people provide both goods and services. For example, in the same garage a man may buy a car or some service, which helps him maintain his car.

What is a vacation? A vacation is a time to leave. In other words, it is an uncertain amount of time in which a person departs from his or her ordinary routine to do something out of the ordinary. So, a vacation is not a two-week break from the office, a ride down a river, or a trip to a new city. Vacations are simply a chance to change for a while, That is why, regardless of their form, we like them so much.

Language is part of culture. Culture, in this sense, means all those customs, skills and attitudes that are part of the behavior of a particular group. The way you live, what you think, and what you want in life are all affected by the culture of the group in which you are raised.

In the simplest terms, a market is the place where seller meets buyer to exchange products for money. Traditional markets still function in many parts of the world. Even in the United States, during summer months, there are farmers’markets where direct selling and buying take place between producers and consumers. Most service industries still operate at this market level.

Manufacturing industries and most agricultural enterprises are more distant from the consumer. Their products pass through several hands—truckers, warehouse workers, wholesalers, and retailers before reaching the final consumer.

Products, or commodities are usually divided into two types: consumer and industrial. Consumer goods are those that are sold to final users, the customers. These goods include food, clothing, automobiles, television sets, appliances, and all those things people go to stores to purchase.

Industrial goods are those that are sold to companies or other businesses for use in manufacturing or other purposes. Automobile makers buy many of the parts used to assemble cars. A tire manufacturer buys rubber, synthetic or otherwise, with which to make tires. Eventually these materials will end up in the hands of final users: the owners of the cars. The nature of industrial goods depends on the nature of the goods to be made for final users. The price of industrial goods and raw materials will influence the price of final goods, those that the consumer buys.

分类法是写说明文的另一种方法，它是将被说明的对象，按照一定的标准划分成不同的类别，一类一类地加以说明。这种方法能将复杂的事物表达得清楚、明白、有条理。

运用分类法时，一定要注意一次分类只能用同一个标准，以免产生重叠和交叉的现象。用分类方法写文章要按照导言——正文——结尾的方式来写。导言用来引出话题并对下文作一简要的概述；正文则要分别解释或分析导言中提出的各个要点；结尾可重申导言的内容或进一步强调全文要说明的核心。请看下面六个例子。

There were two widely divergent influences on the early development of statistical methods. Statistics had a mother who was dedicated to keeping orderly records of governmental units (state and statistics come from the same Latin root, status)and a gentlemanly gambling father who relied on mathematics to increase his skill at playing the odds in games of chance. The influence of the mother on the offspring, statistics, is represented by counting, measuring, describing, tabulating, ordering, and the taking of censuses—all of which led to modern descriptive statistics. From the influence of the father came modern inferential statistics, which is based squarely on theories of probability.

Descriptive statistics involves tabulating, depicting, and describing collections of data. These data may be either quantitative, such as measures of height, intelligence, or grade level—variables that are characterized by an underlying continuum—or the data may represent qualitative variables; such as sex, college major, or personality type. Large masses of data must generally undergo a process of summarization or reduction before they are comprehensible. Descriptive statistics is a tool for describing or summarizing or reducing to comprehensible form the properties of an otherwise unwieldy mass of data.

Inferential statistics is a formalized body of methods for solving another class of problems that present great difficulties for the unaided human mind. This general class of problems characteristically involves attempts to make predictions using a sample of observations. For example, a school superintendent wishes to determine the proportion of children in a large school system who come to school without breakfast, have been vaccinated for flu, or whatever. Having a little knowledge of statistics, the superintendent would know that it is unnecessary and inefficient to question each child; the proportion for the entire district could be estimated fairly accurately from a sample of as few as 100 children. Thus, the purpose of inferential statistics is to predict or estimate characteristics of a population from a knowledge of the characteristics of only a sample of the population.

Sight can be defined in four basic ways. In its most fundamental form, it is the ability to detect the difference between light and darkness. For most simple life forms, from amoebas to earthworms, or for fishes that live in the depths of the ocean, this is about all their eyes can handle.

Far more important to the majority of the animal kingdoms is the second visual skill—the ability to detect movement. This is most important to those animals, such as insects, that must move rapidly to catch food or to flee, and for animals that are often hunted, like mice and rabbits.

Most predatory animals such as hawks and lions rely more on a third skill—form vision, which is the ability to see and recognize the shape or nature of an object. In human terms we measure form vision by the ability to recognize letters on an eye chart. For animals, those forms may become a suitable mate, or a meal, or a predator.

Most of the proposed solutions to the parking problem fall into two broad categories: those that require more land and those that restrict access to prime parking spaces, usually by giving preferred status to faculty, staff, and seniors. As for the first solution, land is in very short supply on the central campus, and, as others have pointed out, using it for parking seems unwise. The other solution, restricted access to parking, displeases the students who will be denied space.

You probably know there are two sides to the brain, the left and the right. According to one theory, the right side deals with the senses (what we see, hear, feel and smell). It is the creative, imaginative side. The left side is concerned with logic. It analyses information and puts it in order. It’s the“educated”side of the brain.

As far as political views are concerned, people fall into three categories. First are the conservative people. Conservatives are opposed to sudden or great changes. As long as they are comfortable, they want to keep things they are. Second are the liberal people. These people are in favor of progress and reform. But their opinions often seem too idealistic. The third type is the moderate people, or “ middle of the roaders” as they are sometimes called. The moderates keep every thing within reasonable limits. They are more practical in this hard world.

Either out of confusion or discomfort we sometimes express our emotions in an unclear way. One key to making your emotions clear is to realize that you most often can summarize them in a few words—hurt, glad, confused, excited, resentful, and so on. In the same way, with a little thought you can probably describe very briefly any reasons you have for feeling a certain way.

In addition to avoiding excessive length, a second way to prevent confusion is to avoid over qualifying or down playing your emotions—“I’m a little unhappy” or “I’m pretty excited” or “I’m sort of confused.” Of course, not all emotions are strong ones. We do feel degrees of sadness and joy, for example, but some communicators have a tendency to downplay almost every feeling. Do you?

A third danger to avoid is expressing feelings in a coded manner. This happened most often when the sender is uncomfortable about revealing the feeling in question. Some codes are verbal ones, as when the sender hints more or less subtly at the message. For example, an indirect way to say “I’m lonesome” might be “I guess there isn’t much happening this weekend, so if you’re not busy, why don’t you drop by?” Such a message is so indirect that the chances that your real feeling will be recognized are slim. For this reason, people who send coded messages stand less of a chance of having their emotions understood—and their needs met.

Finally, you can express yourself clearly by making sure that both you and your partner understand that your feeling is centered on a specific set of circumstances rather than being indicative of the whole relationship. Instead of saying“I resent you,” say “I resent you when you don’t keep your promises”. Rather than “I’m bored with you，” say “I’m bored when you talk about your money. ”

The working conditions were poor. The tables where the workers sat were very high and uncomfortable. Except for a half hour at lunchtime, there were no breaks in the day to relieve the boring work. There was no music. The walls of the workrooms were a dull gray color. I was amazed that the workers hadn’t gone on strike.

Furthermore, the workflow was irregular. There was one especially absent-minded young man in the assembly line who sewed on buttons. After a while I recognized him as “Big Jim,” who used to sit behind me in math class in high school. He was very slow and all the shirts were held up at his position. Workers beyond him in line on his shift had to wait with nothing to do; therefore, a great deal of time and efficiency were lost as Big Jim daydreamed while he worked. All week I wondered why he wasn’t fired.

Einstein was purely and exclusively a theorist. He didn’t have the slightest interest in the practical application of his ideas and theories. His E=mc2 is probably the most famous equation in history—yet Einstein wouldn’t walk down the street to see a reactor create atomic energy. He won the Nobel Prize for his photoelectric theory, a series of equations that he considered relatively minor in importance, but he didn’t have any curiosity in observing how his theory made TV possible.

In order to prevent non-smokers from being affected, measures must be taken to reduce the chances of smoking. A lot of work can be done concerning this. For example, in some public places, such as in theatres, cinemas, and smoking should be forbidden. Even on the train or plane, people should not be allowed to smoke. Doctors, teachers, government leaders should take the lead not to smoke. Above all, the harmfulness and dangers caused by smoking should be made known to all through newspapers, broadcast or TV programes. Also, the growing of tobacco and the production of cigarettes should not be encouraged. Only in this way can we effectively reduce the chances of smoking.

例4的中心思想是要说明为了使不吸烟者免受影响，应尽量采取措施减少被动吸烟的机会。围绕这一中心，作者列举了几方面的例子，如：在公共场所禁止吸烟；医生、教师及公务员等要带头不吸烟；还要让人们了解吸烟的危害等等。

Different people like different sports. Some people choose a sport for the type of exercise it offers. For instance, weight lifting requires a lot of strength, while ballet requires graceful coordination. Other people choose a sport for the type of participation it offers. Basketball, for example, is a team sport where you participate with team members, while running tends to be an individual sport where you do not participate with a team. There are a wide variety of sports to satisfy the interest of many different people.

Telephone is one of the most convenient means of communication. For example, now in big modern cities, there has been a developed telephone system, almost every street has public telephone booth, and more and more households have private telephones. Its network is widespread. You can call anybody freely in the shortest time. Telephone has connected remote places with busy cities and peaceful villages with commercial centres as well. The telephone lines weave a network covering all parts of the world, thus increasing the speed of communication hundreds of time as much as it used to be. As a result, you can ring your relatives and friends everywhere at any time.