A variety of molecular changes typify aging. For example, production of the proteins— collectively known as collagen—in the skin slows down. As the amount of collagen decreases, the skin loses elasticity and begins to sag, bag, and wrinkle. The loss of elasticity can be demonstrated by gently pinching a fold of skin on the back of the hand: The skin of young children will immediately return to form when released, while that of an older person will take longer.