It is normal for the tongue to smell bad when scraped off by hand, which means that the mouth itself has a bad smell. This is because there are food residues in the mouth that often accumulate on the surface of the teeth and are used by bacteria to produce foul-smelling gases that are attached to other structures in the mouth. In addition, the tongue itself is a layer of structure formed by the mixture of filiform papillae and food residues, which are attached to the surface of the papillae and are decomposed by the bacteria in the mouth to produce smelly gas. Because the oral mucosa does not have receptors to sense odor, so although there is odor in the mouth, you can not smell the odor. The tongue scraped off by hand should be brushed well to clean the food residue on the surface of the teeth, and rinse often to keep the mouth clean, so that part of the smelly food residue on the tongue can be removed, and the odor will be reduced.