说明文
说明文是对客观事物的特征、功能和作用等进行说明的文体。要求根据客观事实，如实地逐一予以说明。写作时，可以按时间、空间、逻辑或认识等顺序展开。下面就是一个依照逻辑上的因果关系而展开的说明文。
下面就以一篇习作《青春》来看看这种文章的写作。
①青春是什么? ②对于不同年龄的人，青春又意味着什么? ③就大学生而言，青春是……
On Youth
In the case of college students，youth is just the same as a race in which knowledge competes against time for their sake. During the course of this competition，most students try and invest as much time as possible in the ocean of knowledge，while some yield up the pursuit of knowledge to the enjoyment of pleasures that time offers. It has al-ready been proved that economy of knowledge will feature the century to come，which is a direct challenge to the college students，masters of the new century. If a student realizes the challenge as well as the fact that anyone has youth but once，he or she will never let youth down and help knowledge always hold the lead in the race against time.
【译文】青春
就大学生们而言，青春恰似知识为了大学生们的利益而与时间展开的一场竞争。在这场竞争中，大多数学生千方百计将尽可能多的时间投入知识的海洋，但有一些学生把对知识的追求让位于及时行乐。现已证实，知识经济将是下一个世纪的主要特征;这是对大学生——新世纪主人——的直接挑战。如果一个学生能意识到这一挑战以及一个人的青春只有一次的事实，那他(她)就不会辜负青春，使知识在与时间的赛跑中永远处于领先地位。
