You can find out the answer to this by making a little soap in a greasy frying pan. Put a very little water and a big spoon full of washing (lump) soda in it, and let them boil together. You get a thick soft soap. The grease is broken up and mixed with the soda, and the frying pan is cleaned. Our grandmothers made great kettles full of soft, brown soap by boiling waste fat and wood ashes lye together. Lye, soda and potash, or what are called the alkalies, break up and dissolve the fats. Our skin makes a kind of oil all the time. This oil gathers dust, and makes us and our clothing dirty. We can cut this dusty oil from the skin with soda or some other alkali, dissolved in water. But an alkali is so strong it dries out and breaks up the skin as well as the oil. So we weaken the alkali with other oils or fats first. There is still an excess of alkali in the soap called "free" alkali, enough to take up the oil in our skins and clothing without injuring them. Of course, with the oil comes the dust. If your face smarts or shines after a scrubbing with soap, it is probably because the kind of soap you use has too much "free" alkali in it for your skin. Or the water is hard and does not dissolve the soap properly. As waters and surfaces to be cleaned differ so much, many kinds of soap are necessary.