Hyaluronic acid has been getting a lot of buzz in the beauty world lately. Because of its ability to plump and soften skin, it's been showing up in moisturizers and serums over the last few years. Unlike some other skin care ingredients, hyaluronic acid is actually already present in the human body [Source: WebMD]. A viscous, gooey substance, it helps lubricate joints and is even found in the fluid of eyeballs to help maintain their shape [Source: WebMD]. However, like many substances in our bodies, it naturally depletes as we age—which is why beauty product manufacturers have begun adding it to products. "Hyaluronic acid is a humectant that hydrates by drawing water in to the outer layers from deep within the skin," says Zeichner. In fact, it is said to hold up to 1,000 times its weight in water. As a result, it makes skin appear plumper and feel softer and smoother. That may be why it's often used as a lip filler in plastic surgery!