Every single person wants to get happpiness. But what is happiness？ There may be more than one thousand answers to this question. Happiness is， actually， a satisfying feeling when one"s dreams come true. However， the degree of happiness is largely determined by what kind of person one is and what kind of dream he pursues.
A child who lives in a small and pure world， only dreams of having a cartoon toy or some tasty cookies. So if one day， his parents give him what he wants， his pink cheeks will smile in satisfaction and we"ll know he is very happy now.
For a young person， happiness may become much more complicated since it often involves feeling satisfied after suffering a long and hard struggle. An adult has more dreams than a child and， these dreams， including getting a job promotion， marrying a pretty wife， a fashionable car， and living in a big house， are harder to realize. For the sake of these dreams， every day and night， he has to devote himself to work， burying himself mountains of files. He can not take a holiday， since the desire for the happiness drives him on and on. But what about job satisfaction？ He lives to work rather than works to live. So happiness can not be drived from what he is doing. Even one day when all his dreams come true， he may feel happy just for a moment since this happiness is paid for by many hours of miserable work.
When a person is getting old， when wrinkles spread all over his face， happiness regains the purity it had in his childhood. He finds happiness only during normal activities like doing exercise in the fresh morning， drinking a cup of mellow tea in a warm afternoon， walking a cute dog in a beautiful park and having dinner with the ones he loves.
本文第一段，作者通过定义给出“什么是幸福”，并通过儿童、成人和老人三个年龄段例子，分别说明幸福对于他们的意义。作者在说明“幸福是什么”时分别采用了举例和描写相结合的方法，可见说明文的写作方法是一个综合使用的过程。
(3) 因果说明法（cause-effect）。
因果关系型说明文包含两层意思：一是看到一种现象推断其成因(即由果推因)，另外一个是根据某一现象的特点推测它可能的发展或者是它在某种条件变化下的可能后果（即由因推果）。在写此类说明文时，特别注意回答两个方面的问题：一是事情发生的原因是什么，二是事情发生后会带来什么影响。当分析事情的原因时，往往可能涉及多个原因，此时应分清主次、先后，且各原因之间有所关联，不能“各自为政”，相互脱节。
范文：
College Students" Expense Are High
College student expenses are currently getting higher and higher. According to a college student"s Monthly Expenses Survey， which covers most regions of the whole country， over 40% of the college students" average monthly expenditure can reach 800 yuan. For students who study in the developed cities like Beijing， Shanghai and Shenzhen， the number is even higher， sometimes exceeding 1000 yuan.
What is causing this increase in college students" daily expenditures？ Modern college students are living in an age of consumer abundance， luxury goods are readily available. Due to this availability， they face much greater temptation. In addition， the development of science and technology brings us some new thins such as telephone， cell phone， computers， the internet and MP4 players， paying for which also costs college students a lot of money. They can find all kinds of excuses for using these new thins. Another factor which can not be overlooked is that Chinese parents are clueless when it comes to making the children financially independent. American college students typically spent all their summer holidays and weekends earning money supporting them in college because their parents regard them as adults. By contrast， Chinese college students would rather ask their parents for money and their parents don"t hesitate to give them money.
The face is， in today"s China， the average income of a family in urban area is about 1500 yuan per month， and， so the expenses of college students are obviously high. The imbalance between the family"s low income and the college student"s high expenditure should attract more attention. Both the parents and the college student need to understand the importance of financial independence.
这篇文章是一篇解释大学生消费高的原因的说明文。文章第一段第一句点明了主题，并通过数据进一步支持主题。第二段，作者主要从高消费社会、科技的发展及中国学生习惯向父母伸手等分析为什么大学生消费高。第三段，作者指出此类问题需引起重视。注意在分析原因时要避免只在某一原因范畴内打转，不能很好地拓展开来。
(4)分类法（classification）。
分类说明文是将事物按照一定的标准和范围，系统地分成若干点或若干方面来说明。英语中可用divide（是指把完整的事物分成不同的部分）和classify（是指把不同的事物依据它们之间某些相同的特点进行归类）两个动词来表达。分类法是对属于同一类别的事物，根据其特质进行分类，然后进行说明，其目的是使文章系统条理。写分类说明文时特别要注意分类标准要统一，不要重复分类。
范文：
There are many beer ads on TV. Beer makers have pounded audiences with all kinds of campaigns to sell beer. Each type of as， however， seems to be targeted toward a different economical level of the TV viewing audience.
The first type of ad appeals to workingclass people. The as shows the “boys” headed down to the neighborhood tavern after a tough day on the job at the auto plant or the construction site. The waiter congratulates them on a job well done and encourages them or rewards themselves with beer.
The second kind of ad aims at an upper-middle-class. The actors in these ads are shown in glamorous or adventure settings.
The third type of ad appeals to people with a weight problem. These are ads for the light beers， and they use sports celebrities and indirect language to make their points.
To the TV audience， it may well seem that beer is necessary to every adult"s life. After all， we need it to recover from a hard day at work.