Youthful skin looks and feels great mainly due to its high content of water. Day-to-day external stressors, such as UV exposure, plus just the normal ageing process lead to loss of moisture. And the key molecule responsible for retaining water in our skin is hyaluronic acid (HA).
When it comes down to cosmetic moisturizers, they follow the natural hydrating mechanism of the skin: They should add water to the skin and bind it there.