Also known as glycerol, glycerin is a humectant that works similarly to hyaluronic acid, says Zeichner. Composed of fats and sugars, it's a sweet-tasting lipid that's actually found in all fats, whether they are from animals or vegetables [Source: Paula's Choice]. Because glycerin is a humectant, it readily absorbs and retains water, keeping skin sooth and hydrated. It attracts water from the environment and from the lower layers of skin (aka, the dermis) adding moisture to the surface layers of skin (the epidermis). In general, it helps maintain the skin's outer barrier, which helps prevent dryness and flaking. One recent study found that after 10 days of treatment with a cream that was 20 percent glycerin, volunteers saw an increase in corneometer values, a measurement used to measure the hydration levels of your skin [International Journal of Cosmetic Science].