Yeast is a living organism. It grows and reproduces itself. The yeast cells have special sacs, called spores, which protect them from drying out. The spores are dormant until conditions are right for them to germinate. The yeast must reproduce itself if it is to remain viable. A small amount of the yeast is mixed with water and flour. The mixture is allowed to ferment. The fermentation process produces carbon dioxide gas. The gas bubbles are trapped in the small, air-filled sacs. The yeast is then removed from the mixture.