A protective layer is a barrier that helps to protect something from damage or harm. OZONE is a form of oxygen that is found in the Earth's atmosphere and acts as a SHIELD against ultraviolet radiation. A LINER is a thin layer of material used to line the inside of something, such as a garment or container, for protection or insulation. SKIN is the outer covering of an organism, which provides protection from the environment. SEALER is a substance used to fill in gaps and seal surfaces, such as to waterproof or protect from corrosion. Finally, a SHIELD is a device or structure used to protect against physical, electrical, or other forms of attack. All of these terms refer to different types of protective layers.