Dan, different ingredients weigh differently by volume. A cup of water equals 8 ounces. But a cup of feathers on a scale might weigh 1/2 ounce. The only ingredients that are heavier than water by volume are invert sugars: honey, molasses, maple syrup. Flour, being a powdery substance, is properly measured by stirring it to aerate it, sprinkling it into a flat-topped measuring cup, and sweeping the excess off the top. When measured this way, flour weighs 4 1/4 ounces per cup. The idea with sourdough is to have equal amounts by weight of water and flour. —Susan

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