历年四级考试中的“How I Overcome Difficulties in Learning English”、“My Ideal Job”以及图表作文的段都属于说明文的范畴，如，2002年6月的根据坐标图写题为“Students Use of Computers”的作文。

　　说明文必须按一定的顺序进行说明：时间顺序、空间顺序、逻辑顺序或认识顺序，选择具体的特点和细节来说明。

　　所以，说明文的思路要围绕说明顺序、事物的特点和细节展开。例如：

　　范文点评

　　A Famous Clock

　　When you visit London， one of the first things you will see is Big Ben， the famous clock which can be heard all over the world on the B.B.C. If the Houses of Parliament had not been burned down in 1834， the great clock would never have been erected. Big Ben takes its name from Sir Benjamin Hall who was responsible for the making of the clock when the new Houses of Parliament were being built. It is not only of immense size， but is extremely accurate as well. Officials from Greenwich Observatory have the clock checked twice a day. On the B.B.C. you can hear the clock when it is actually striking because microphones are connected to the clock tower. Big Ben has rarely gone wrong. Once， however， it failed to give the correct time. A painter who had been working on the tower hung a pot of paint on one of the hands and slowed it down!

　　【点评】这是《新概念英语》中的一篇经典说明文，介绍了世界的“大本钟”。文章开篇指明该钟位于英国伦敦，且全世界的人们通过BBC就能够听到钟声。接下来讲述了“大本钟”的由来，特点——不仅外型巨大，而且走时准确，在结尾处讲了“大本钟”的一个幽默的小插曲——油漆工把油漆桶挂在指针上，把钟弄慢了。细读这篇文章，我们不难发现，区区159个单词就把“大本钟”的光辉形象牢牢地刻在读者的脑海里，为什么?本文基于事实，分析了主题的来龙去脉，语言简洁又不失生动贴切，作者态度客观又充满幽默，使读者既“明白了事理”，又对事物有较完整明晰的了解认识，此乃说明文之真谛也。

        以“Goals”为题打开一篇说明文的思路。

　　参考思路

　　1)目标的分类：(可根据时间顺序，空间顺序、逻辑顺序或认识顺序，任选一个进行分类和排列)

　　2)各个类别的特点和细节：

　　3)组织成文：

　　范文点评

　　Goals

　　Everyone has goals. Many people do not reach their goals for lack of planning. These three sets of goals should be developed： lifetime， intermediate and short term.

　　Lifetime goals should be written out. These goals should be clear and reasonable. For instance， the goal to win a research prize is clear and measurable. Other lifetime goals might include publishing a certain number of influential books or articles， earning a departmental chairmanship at a major university， successfully competing for a grant from the National Science Foundation or securing a consultantship with a big corporation.

　　Intermediate goals relate to outcomes during the next few years. For the beginning doctoral student， intermediate goals may include completion of course work and preparation of a doctoral research proposal. Like lifetime goals， these are measurable and clearly stated.

　　Short-term goals involve outcomes expected during the next 12 months. These goals are more performance oriented， and they help with achievement of intermediate goals. Examples of short-term goals are earning an A in a course， learning how to program a computer or performing an experiment.

　　【点评】本文介绍了人生的三种目标：终生的、长期的和短期的目标。在文中采用了说明文常用的方法：定义、举例、分类和比较等。