Koala bears aren't really bears. Jellyfish aren't really fish. And moisturizers don't really add water to the skin. And it's a good thing the skin doesn't absorb water or we'd swell up like a sponge when we swam in the ocean. And we don't.
Built like a brick wall, the outermost layer of skin is an incredible barrier. It protects us from outside elements including bacteria, fungus, mold, and ultraviolet radiation from the sun. This barrier makes us virtually waterproof. One of the most important functions of skin is to prevent the body from losing water. Water is constantly evaporating from the skin into the environment. On cold winter days our skin feels drier as we lose more water into the air, on humid days we lose less.
Effective moisturizers do not add water to the sin. They create an environment where water loss from the skin is minimized. By decreasing the amount of water loss, skin water content can rise. Well formulated "moisturizers" contain occlusives, ingredients that prevent water evaporation by forming a film on the skin surface. Petroleum jelly is the gold standard occlusive decreasing water loss from the skin by 98%. Although it is extremely effective as an occlusive, it can feel greasy on the skin, not always cosmetically elegant for the user. Silicone derivatives like dimethicone are often used in lieu of petroleum jelly or in addition to it to improve the elegance of the product. These synthetic silicone derivatives, however, are not as effective in preventing water loss from the skin. Effective moisturizers contain other ingredients called humectants that help the skin hold onto water. Glycerin and hyaluronic acid are common humectants.
So the next time you use your favorite moisturizer, do know that the product will not add water to your skin. If it is an effective product, however, it will prevent water loss from your skin and create an environment where your skin can hold onto it's own water. Moisturizers don't really moisturize. The skin does that on its own.
By the way, fireflies aren't really flies. And strawberries aren't really berries.