Writing is a form of communication. Its structure is linear, like a path. That means the reader intakes the first word, remembers it, and then intakes the next word. As the reader progresses through the words, sentences, and paragraphs, the remembered bits of information coalesce into a mosaic. The author–the person who controls the flow of information–slowly guides the reader along the path, showing the reader one thing after another.

Each new thing shown to the reader becomes part of the mosaic until a complete picture emerges. This system is used to both convey information to the reader and, in many cases, to build an emotional response. For example, in a novel, we meet the protagonist, learn more about their experiences, and (usually) develop an affection for them. We become emotionally invested in their predicament.

After words and sentences, the paragraph is the third largest building block of writing. In expository writing, the paragraph is a mini-essay. It introduces and elaborates upon an idea or variation of an idea. In terms of the role of memory, it tells the reader how to categorize the information that follows.

For example, if the first or topic sentence of the paragraph is, “The house was painted red,” the author is promising the reader that the information in the following sentences will elaborate on this statement. The author will talk about the house and its various features, and set the stage for action to follow.

If the topic sentence is, “The killer, knife in hand, leered at his victim,” we’re going to be shown a scene of tension or violence directly related to these nine words.

It’s the author’s choice as to how to allocate his or her text into paragraphs. Paragraphs can be any length–even just one word. After all, writing is a form of art, and the rules, once learned, are flexible. But every writer needs to remember that your reader has allowed you to take them by the hand and lead them along the winding path, and while you can surprise your reader, you never want to lose them!