Why does bread rise when you add yeast to dough?

Yeast are living microorganisms that feed on the sugars present in flour. As they consume these sugars, they produce carbon dioxide gas and alcohol through a process called fermentation. The carbon dioxide gets trapped within the gluten network that forms when flour and water are mixed and kneaded together. Gluten is an elastic protein structure that stretches and holds the gas bubbles in place, causing the dough to expand and rise. When the bread is baked, the heat kills the yeast, stops fermentation, and causes the trapped gas bubbles to expand further before the gluten structure sets permanently. This is what gives bread its light, airy texture with those characteristic holes throughout the crumb.