The document discusses the key components of paragraphs, including the topic sentence. It explains that a topic sentence contains a topic and controlling idea. The topic tells what the paragraph is about, while the controlling idea tells what the paragraph will say specifically about the topic. The document provides examples of topic sentences and how to identify the topic and controlling idea within them. It notes that while the topic is usually first, the controlling idea can come first in some cases. The document aims to help readers understand how to identify the essential elements of a strong topic sentence.