Bread is made by mixing flour, water, yeast, salt, and other ingredients together to form a dough. Yeast is a type of microorganism that ferments the sugars in the dough, producing carbon dioxide gas that causes the dough to rise. The dough is then baked in an oven, which causes the carbon dioxide gas to expand and creates air pockets in the bread. The heat from the oven also causes the crust of the bread to form.