Cheerios are made from oat flour and some other ingredients, such as sugar, salt, and starch. The dry ingredients are mixed with water and formed into small, hard circles by cooking and forcing through dies. The small circles are partially dried and then puffed by heating with steam in a cylinder under pressure, then allowed to release from the high pressure to atmospheric pressure. This causes the circles to expand as water inside turns to steam. The puffed pieces are further dried, then vitamins and minerals are sprayed on and they are dried again.