What Are Those Ingredients in Your Skin-Care Products?

Hyaluronic Acid

Also known as: Sodium hyaluronate.

Good for: All skin types, especially dry skin.

What it treats: Aging.

How it works: Hyaluronic acid attracts and seals water into the skin. "When you hydrate the skin from outside, you make fine lines less noticeable," says Susan Weinkle, an assistant clinical professor of dermatology at the University of South Florida, in Tampa. Avoid hyaluronic acid if you live in a low-humidity climate. It will sit unabsorbed on the surface of your skin and may actually dehydrate it, drawing out moisture.

RS pick: L'Oréal Skin Genesis Daily Moisturizer Lotion, $25 at drugstores.