Using a sourdough starter is an ancient method of beginning to make a leavened bread. The starter consists of small amounts of some of the basic bread ingredients, such as flour, water, milk and, in this recipe, yogurt. This mixture is left out to attract wild yeasts from the air. The yeasts feed on the starch in the flour, resulting in the fermentation and souring of the mixture and creating a base to which additional ingredients can be added to make a dough. The starter may be left to develop for many days, depending on the desired degree of sourness.