Bread rises because yeast is a living organism that feeds on the sugars in flour. A it fe it rele carbon dio, and t gas coll in thou of sm pockets throu the do. Gluten, a network of proteins formed when flour is mixed with water, stretches around each pocket and holds the gas in place. Heat finishes the process: the trapped bubbles expand, the yeast dies, and the gluten sets into a firm structure that keeps the shape the gas gave it.

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