Because it is coated on the surface with a layer of adhesive. The earliest adhesives came from animals and plants. In the 19th century, rubber was the main ingredient of adhesives; modern polymers are widely used. Adhesives can stick to things because of the formation of bonds between their molecules and the molecules that they are attached to, which bonds the molecules firmly together. Adhesive ingredients, according to different brands, different types, there are a variety of different polymers.