Place the quarter-cup of tepid water in a small bowl and sprinkle in the yeast. When it dissolves, add the third of a cup of water.
2
Beat in the flour until the very soft dough is smooth and elastic. Cover tightly and refrigerate for at least 24 hours. The sourdough will remain useable for about five days.
3
To keep the sourdough alive, remove and use or discard a half-cup of it. Beat in a third of a cup of water and a half-cup of flour. Cover tightly and refrigerate.