The main grain used in making beer is barley. Barley serves as the primary ingredient in the brewing process due to its high starch content, which is essential for fermentation. During brewing, barley grains are malted, soaked in water, and then heated to activate enzymes that convert starches into sugars. These sugars are then fermented by yeast, producing alcohol and carbon dioxide, resulting in beer. While barley is the most common grain, other grains like wheat, corn, and rice can also be used in brewing, leading to diverse beer styles.