Sourdough bread is a type of bread made through a natural fermentation process. It uses a mixture of flour and water to create a starter, which contains wild yeast and bacteria. This starter is what gives sourdough its distinctive tangy flavor and chewy texture. The fermentation process also helps improve the bread's nutritional value and shelf life. Unlike commercial bread, which often uses added yeast, sourdough relies on the natural microorganisms present in the environment. This traditional method of baking has been practiced for thousands of years and is popular in many cultures around the world, including France and San Francisco.