Why does bread dough rise when yeast is added?

Yeast are living microorganisms that consume sugars in the flour through a process called fermentation. As they digest these sugars, they produce carbon dioxide gas and ethanol as byproducts. The carbon dioxide gets trapped within the gluten network that forms when flour and water are mixed and kneaded. Gluten is an elastic protein structure that stretches and holds the gas bubbles in place, causing the dough to expand and rise. The more the yeast ferments, the more gas is produced, and the larger the dough becomes. When the bread is baked, the heat kills the yeast, the alcohol evaporates, and the expanded structure sets permanently, giving bread its light and airy texture.