Humectants. Theoretically, humectants pull water into the stratum corneum both from the air and from deeper layers of the skin. However, when the humidity is low, there's so little water in the air that almost all of the water comes from the inside out. Some of the commonly used humectants include glycerin, honey, panthenol (or vitamin B5, an example of a vitamin being used for its physical, not nutritional, properties), sorbitol (which we are used to seeing as an artificial sweetener), and urea. Humectants can potentially make skin even drier by pulling water into a damaged, arid stratum corneum that doesn't hold moisturizer. So, as a practical matter, they are almost always used with occlusive ingredients that trap the moisture the humectants draw into the stratum corneum.