What causes bread dough to rise when baking?

When yeast ferments the sugars in flour, it produces carbon dioxide gas as a byproduct. This gas gets trapped within the gluten network that forms when flour proteins hydrate and are worked together. The trapped bubbles cause the dough to expand and rise before and during baking. In the oven, heat accelerates this gas production briefly, then kills the yeast. The heat also causes the gas bubbles to expand further and the gluten and starch structure to set permanently, locking in the risen shape. Breads made with chemical leaveners like baking soda or baking powder rise through a similar mechanism, but the gas comes from a chemical reaction rather than biological fermentation.