Yeast starters were commonly used before packeted yeasts and other leaveners as we know them today were commercially available. A yeast starter would be a mixture of water, flour, and sugar, (today commercial yeast are often mixed in) which is allowed to ferment, capturing natural airborne yeasts. When the mixture has fermented, a portion is used in a recipe, and the amount taken is replenished with equal amounts of water and flour. This process can be kept going indefinitely. Real sourdough bread is one of the most popular breads using this method.