A topic sentence is the first sentence of a paragraph. In the first paragraph of a text, it does not have to be the first one. It introduces the reader to the topic of the paragraph and, by extension, of the essay. It should be clear and concise, as well as attention-grabbing.
Below, there is an expository paragraph on the importance of reading. Notice that the topic sentence shortly states what the paragraph is about and the supporting sentences coherently link to it.
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It is a commonly acknowledged truth that reading is vitally important for both children and adults. The most obvious reason for the significance of reading often is to learn. When you read, you acquire information and tend to store it longer. Besides that, reading exercises the brain through stimulation. That plays a crucial role because of its subsequent benefits, namely improving focus, concentration, memory, and communication skills. Furthermore, as well as entertaining, reading can reduce stress. Research has proven Cortisol, the stress hormone, to be greatly decreased after reading. Taking a walk is less effective than reading fiction when it comes to lowering stress levels. Lastly, by exposing individuals to alternative points of view, reading enhances their critical thinking skills.
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The topic sentence states what the paragraph is about (the significance of reading) and the subsequent sentences elaborate on it, providing relevant information to prove it to be true in this case.