1 Don't be afraid of the expository essay. Expository is just a fancy word that means explaining a subject/topic. An expository essay, then, is a written explanation of a subject. The goal is to share information with the reader. It may be abstract or concrete in nature. That is, it may be about an idea (the U.S. system of checks and balances) or about a real take-a-photo-of-it subject (making a toothpick model of the Golden Gate Bridge).
2 Your expository essay will exhibit your knowledge of a subject. That knowledge may be familiar to you already, or it may require research. It will be fact-based and not opinion-based. It will be without first and second-person pronouns. Focus your attention on the topic and not yourself or the reader.
3 Explanations can be presented in a variety of methods, such as the following:
Explain a step-by-step process
Compare and/or contrast two items
Explain with examples
Divide and classify
Identify a cause-effect relationship (causal analysis)
4 The first method requires sequential order in the details. If you were to write the instructions for making a peanut butter and jelly sandwich, laying out the bread would come before spreading the peanut butter and jelly, which comes before putting the two slices of bread together--sticky side in, of course! Methods #2 through #4 employ an order of importance; the two options are to arrange the information from most-to-least important (news reporting style) or least-to-most important (dramatic buildup). The method of causal analysis involves one of two options: identify a cause and predict its effect, or present the effect and identify its cause. For example, if the effect is that Hammy is dripping wet, the cause might be that he got caught in the rain or sprayed with a garden hose. It's detective work.
5 As you organize your paper, keep these guidelines in mind. Whether it takes just one paragraph or several to present your explanation, no paragraph should have less than three sentences. Each will need a topic sentence followed by two or more supporting sentences. Don't assume that the reader is familiar with any portion of your subject. Neither should you choose big, complicated words when a simple one will work. It is better to be understood than to appear erudite. (You don't know the meaning of erudite? That's the point! Showing off by used big words is annoying! Don't do it!) You should also avoid overuse of the word "then," as in "Then I went to school, and then I went to lunch, and then I went to recess."