Yeast is the ingredient in bread that makes it rise. Specifically, the process involves the fermentation of sugars by yeast, which produces carbon dioxide gas and alcohol. This gas gets trapped in the dough's gluten structure, causing it to expand and rise. The heat from baking further causes the gas bubbles to enlarge, resulting in a light, airy texture in the finished bread. Different types of yeast, such as active dry yeast and instant yeast, can be used, but all serve the same purpose of leavening the dough.