After identifying the topic of a paragraph, you need to find the main idea, which is the writer’s position or viewpoint about that topic. For example, in the following paragraph, the topic is “carbon dioxide,” and the main idea is “carbon dioxide is a product of burning.” The underlined first sentence is main idea, but for brevity’s sake, it can be paraphrased or articulated in your own words.

When any fuel, such as coal, gas, oil, or wood, burns, it sends forth gases into the surrounding atmosphere. These gases, like air, are invisible, and were unknown to us for a long time. The chief gas formed by a burning substance is called carbon dioxide (CO₂) because it is composed of one part of carbon and two parts of oxygen. This gas has the distinction of being the most widely distributed gaseous compound of the entire world; it is found in the ocean depths and on the mountain heights, in brilliantly lighted rooms, and most abundantly in manufacturing towns where factory chimneys constantly pour forth hot gases and smoke.