While drinking eight glasses of water a day is recommended to stay healthy, it's not going to fill in your wrinkles, according to the University of California, Berkeley.

That's because the outermost layer of skin cells is the oldest. And since those cells are already dead, they're not absorbing moisture from within.

The moisture in your skin depends mainly on external factors, including the drying effects of soap and bathing, the air, the sun, and indoor heating.