Dear Dr. Boli: What do all those additives and special ingredients in my shampoo do?—Sincerely, A Balding Man.

Dear Sir: Shampoo, in its most fundamental form, is a species of detergent. It is the detergent that cleans your hair. All those other things—the herbal ingredients, the scents, the vitamins, the minerals, the conditioners, and so on—are not detergent. In fact, those ingredients, from the point of view of getting your hair clean, are, technically speaking, dirt. The key measurement in any shampoo, therefore, is the Dirt-to-Detergent Ratio, or DDR. As long as the DDR is low enough, the shampoo will still clean your hair. If the DDR is too high, the extra herbs and spices in the shampoo will add more dirt to your hair than the detergent can clean out, and your hair will actually become dirtier when you wash it. In general, the more expensive the shampoo, the higher the DDR. Dr Boli himself washes his hair, including his celebrated beard, with dishwashing liquid.