You use soap every day (well, hopefully) and yet very few of us know how it actually works to keep us squeaky clean. The answer? All soaps are derived from fats that emulsify oil—meaning they make oily substances float away in water—and the type of fat the soap contains determines its quality. In its simplest form, soap is fat mixed with lye, but modern soaps add chemical components with perfumes and dyes. The type you prefer is up to you, of course, but simple soaps tend to be best for anyone concerned with skin irritations and allergic reactions. And to think we always assumed a soap is a soap is a soap.