Bakers use yeast to turn flat, sticky dough into tall, fluffy bread. The yeast interacts with sugar and fills the bread with pockets of air as it bakes.
When you buy yeast for baking cinnamon rolls or rye bread at home, it comes in a small packet and looks like little brownish grains. The first step in using yeast is often mixing it with warm water and a little sugar. When yeast is active, or alive, the mixture quickly becomes bubbly and foamy. Yeast is also used in brewing beer and wine. The word yeast comes from a Germanic root — the German word Gischt, or "froth," is closely related.