This exercise is designed to give students practice in creating and understanding topic sentences. Rather than asking students to create their own paragraphs headed with topic sentences, this exercise gives students the paragraphs and asks them to synthesize the topic sentences from the content provided. Such back-formation can help students grasp that a topic sentence does not merely start the paragraph, but also organizes and summarizes its key content.
Gleason, Dan, "The Headless Paragraph: Back-forming Topic Sentences" (2011). Writing Bootcamp Unit. 1.