Bread is one of humanity's earliest and most important foods, basically comprising baked 'dough' – a mixture of flour and water. In developed western societies, wheat flour is most commonly used and the dough is 'leavened' (i.e., increased in volume by introducing small bubbles of carbon dioxide throughout) using yeast. In making bread, the chosen blend of flours is mixed with water, yeast, shortening, and salt (and sometimes sugar and milk) to form the dough. This is then kneaded to distribute the gluten throughout the mix, left to rise, kneaded again, molded into shape and left to rise a second time before baking. Bread is generally high in carbohydrates though low in protein. The vitamin and mineral content depends on the ingredients and additives used.