As the readers move into a paragraph, they need to know where they are in relation to the whole essay, and what they should expect to read in the sentences to come. Topic sentences often act like tiny thesis statements. Like a thesis statement, a topic sentence makes a claim of some sort. As the thesis statement is the unifying force in the essay, so the topic sentence must be the unifying force in the paragraph. Also like thesis statements, when the topic sentence makes a claim, the paragraph that follows must expand, describe, or prove it in some way. Topic sentences make a point and give reasons or examples to support it.
Often topic sentences will be working in conjunction with transitional elements to make the move from one paragraph to the next smoother. (See the Writing Center's handout on transitions for suggestions and ideas.)
For example, consider the following topic sentence:
Many fast-food chains make their profits from adding a special ingredient called "secret sauce" to their foods.
If this sentence controls the paragraph that follows, then all sentences in the paragraph must relate in some way to fast food, profit, and "secret sauce":
Made largely from edible oil products, this condiment is never listed on the menu. In addition, this well-kept industry secret is the reason why ingredients are never listed on the packaging of victuals sold by these restaurants. "Secret sauce" has a chemical property which causes temporary amnesia in consumers. After spending too much money on barely edible food bereft of any nutritional value, most consumers swear they will never repeat such a disagreeable experience. Within a short period, however, the chemical in "secret sauce" takes effect, and they can be depended upon to return and spend, older but no wiser.
Note that the paragraph finishes by "proving" the claim contained in the topic sentence, that many fast-food chains make their profits from adding a special ingredient called "secret sauce" to their foods.This is important – make sure you revisit your topic statement in your concluding sentence to establish that you've verified your point. Word your statement differently, though, to avoid being repetitive.
Example courtesy of Turner, Dorothy. "Writing Topic Sentences." The University of Ottawa Hypergrammar. 1996. 2 April 2005 <http://www.uottawa.ca/academic/arts/writcent/hypergrammar/ partopic.html.>