Because it is coated with a layer of adhesive on the surface. The first adhesive from animals and plants, in the nineteenth century, rubber adhesive is the main ingredient; while modern is widely used various polymers. Adhesive can stick to something that is due to the formation of bonds between the molecules and the molecules themselves to be connected items, such molecules can be bonded firmly sticking together. Adhesive ingredients, according to different brands, different types, there are a variety of different polymers.