There is a thin layer of white moist tongue coating on the back of normal people's tongue, which is called tongue coating. It consists of exfoliated keratinized epithelium, saliva, bacteria, food scraps and oozing white blood cells. Under normal circumstances, due to chewing and swallowing, as well as saliva and diet washing, the substances on the surface of the tongue are often constantly removed, leaving only a thin layer of white tongue coating. When sick, eating less or only soft food reduces chewing and tongue movement, or saliva secretion decreases and tongue coating thickens. Common tongue coatings are white, yellow and black. Traditional Chinese medicine believes that tongue coating is caused by stomach qi, and cold and heat of diseases can be distinguished from white and yellow tongue coating; The severity of diseases can be distinguished from the thickness of tongue coating; The transformation of diseases can be distinguished from the changes of tongue coating.