The moisture level in your skin is affected by environment (low humidity or harsh winds) and age (oil production slows over time), but not by how many trips you make to the watercooler. "You can hydrate the skin only from the outside, with moisturizer," says Rodan. "Skin isn't like a plant that wilts and then perks up with a drink of water." If you became extremely dehydrated (usually as a result of illness), your skin could begin to look sallow. (You'd also probably start to feel weak and dizzy.) "But as long as you respond to your thirst, your body will be maximally hydrated," says Dennis Gross, MD, associate clinical professor of dermatology at New York University Medical Center.