[originaltext] Different kinds of writing achieve different purposes. On the

Different kinds of writing achieve different purposes. On the basis of controlling purpose we traditionally divide all prose into three categories: narration, description and exposition. Of these, exposition is especially important to the college student since much of what he reads, and most of what he writes, is expository prose. Exposition is writing that explains. In general, it answers the questions how and why. If we go into any university library, most of the books we find on the shelves are examples of exposition. Philosophies, histories, literary essays, theories of economics, studies of government and law, the findings of sociology, the investigations of science—all these, however different, have for their purpose to explain. It is the most common kind of writing, the sort with which we conduct our workaday affairs—the business letter, the doctor’s case study, the lawyer’s brief, the engineer’s report and so on.
    On paper, the writing of exposition begins with paragraphs. Within each paragraph, the writer shapes and develops a single unit of thought. Generally, paragraphs of exposition contain two different kinds of statements. The first—a general, rather abstract statement—is called a topic sentence. The second class of statements consists of particular facts, examples, illustrations and supporting details. Most often the topic sentence stands first in the paragraph unless one or two sentences of transition go before.