In a covered 4-qt. glass or ceramic container, mix flour and yeast. Gradually stir in warm water until smooth. Cover loosely with a kitchen towel; let stand in a warm place 2-4 days or until mixture is bubbly and sour smelling and a clear liquid has formed on top. (Starter may darken, but if starter turns another color or develops an offensive odor or mold, discard it and start over.) , Cover tightly and refrigerate starter until ready to use. Use and replenish starter, or nourish it, once every 1-2 weeks. To use and replenish starter: Stir to blend in any liquid on top. Remove amount of starter needed; bring to room temperature before using. For each 1/2 cup starter removed, add 1/2 cup flour and 1/2 cup warm water to the remaining starter and stir until smooth. Cover loosely and let stand in a warm place 1-2 days or until light and bubbly. Stir; cover tightly and refrigerate. To nourish starter: Remove half of the starter. Stir in equal parts of flour and warm water; cover loosely and let stand in a warm place 1-2 days or until light and bubbly. Stir; cover tightly and refrigerate.