1 package active dry yeast
          2 cups warm water
          2 tablespoon vinegar
          2 cup all-purpose flour

          Using a stone jar or crock, dissolve the yeast in the warm water. Add the
          vinegar. Stir in the flour. Let mixture stand in a warm place for 3 to 4
          days or until it is bubblly and smells sour. Refrigerate it.