Hyaluronic acid is a naturally occurring substance. It is a key component of the skin's extracellular matrix, which is responsible for maintaining the skin's structure and hydration levels. Hyaluronic acid is able to hold up to 1,000 times its weight in water, making it a highly effective hydrating agent for the skin. When applied topically, hyaluronic acid is able to penetrate the skin and bind to water molecules, helping to hydrate and plump the skin. It also helps to promote collagen production, which can improve the elasticity and firmness of the skin.