Most leavened breads are made with yeast, a microscopic organism that feeds on carbohydrates in flour, converting them into alcohol and carbon dioxide in a process called fermentation.
Breads made with yeast must be allowed time to rise before baking. Bakers set the dough aside in a warm, moist environment; this enables the yeast to multiply, producing more carbon dioxide gas during fermentation. The Carbon dioxide gas causes the dough to rise.