Hyaluronic acid is a humectant, which means it attracts water into the skin and plumps it with hydration. “Think of the skin as an actual sponge,” says board-certified dermatologist Ava Shamban, M.D., founder of SKINFIVE, about the ingredient category. “When it is dry, it is thin, brittle, not pliable, rough in texture, dull, and sallow in color. Run it under water, and it is instantly plump, dense, and smooth. Even the color is brighter.”