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Imagine each paragraph as a sandwich. The real content of the sandwich—the meat or other filling—is in the middle. It includes all the evidence you need to make the point. But it gets kind of messy to eat a sandwich without any bread. Your readers don’t know what to do with all the evidence you’ve given them. So, the top slice of bread (the first sentence of the paragraph) explains the topic (or controlling idea) of the paragraph. And, the bottom slice (the last sentence of the paragraph) tells the reader how the paragraph relates to the broader argument. In the original and revised paragraphs below, notice how a topic sentence expressing the controlling idea tells the reader the point of all the evidence.

Original paragraph
Piranhas rarely feed on large animals; they eat smaller fish and aquatic plants. When confronted with humans, piranhas’ first instinct is to flee, not attack. Their fear of humans makes sense. Far more piranhas are eaten by people than people are eaten by piranhas. If the fish are well-fed, they won’t bite humans.

Revised paragraph
Although most people consider piranhas to be quite dangerous, they are, for the most part, entirely harmless. Piranhas rarely feed on large animals; they eat smaller fish and aquatic plants. When confronted with humans, piranhas’ first instinct is to flee, not attack. Their fear of humans makes sense. Far more piranhas are eaten by people than people are eaten by piranhas. If the fish are well-fed, they won’t bite humans.

Once you have mastered the use of topic sentences, you may decide that the topic sentence for a particular paragraph really shouldn’t be the first sentence of the paragraph. This is fine—the topic sentence can actually go at the beginning, middle, or end of a paragraph; what’s important is that it is in there somewhere so that readers know what the main idea of the paragraph is and how it relates back to the thesis of your paper. Suppose that we wanted to start the piranha paragraph with a transition sentence—something that reminds the reader of what happened in the previous paragraph—rather than with the topic sentence. Let’s suppose that the previous paragraph was about all kinds of animals that people are afraid of, like sharks, snakes, and spiders. Our paragraph might look like this (the topic sentence is underlined):

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