A sourdough starter is a mixture of flour and water that captures wild yeast and bacteria from the environment. This natural fermentation process creates a bubbly, tangy culture that can be used to leaven bread. Unlike commercial yeast, a sourdough starter develops unique flavors and textures in baked goods. To create a sourdough starter, you typically combine equal parts flour and water and let it sit at room temperature. Over several days, you feed it with more flour and water, encouraging the growth of yeast and bacteria. Once active, it can be used in various recipes, including sourdough bread.