The tooth is made up of four layers: enamel, dentin, bone and pulp. Enamel is a layer of milky white translucent tissue with hardness second only to diamond. However, there is a kind of bacteria that can destroy our teeth and form cavities, i.e. “worm teeth”, which is Streptococcus pyogenes. When Streptococcus pyogenes mixes with mucin and food residues in saliva, it forms plaque and sticks firmly to the surface of teeth. It will produce an acid, that is, lactic acid, after fermentation reaction with sugar and starch in food residue, which will erode tooth enamel to form caries, according to the degree of caries can be divided into shallow caries, medium caries and deep caries, the difference between the three is as follows: 1, shallow caries: only the crown part is damaged, the color of tooth surface will change, showing chalky or yellowish brown, there is no cavity formation or slight shallow sulcus point gap caries in this period. 2.Middle caries: there is obvious caries cavity formation, from enamel to superficial and middle layer of dentin, most patients have no conscious symptoms, but occasionally sensitive to cold, hot, acid and other stimulus, when the stimulus disappears, the symptoms will disappear immediately; 3.Deep caries: injured to middle and deep layer of dentin, close to pulp cavity or already injured to pulp, the affected tooth has obvious deeper caries cavity, can keep food residue, to cold, hot, acid and other There is obvious reaction to cold, hot and acid stimulation. After the cavity is formed, the affected tooth should be treated in time, and the daily cleaning work can prevent or slow down the further development of caries. For example, brush the teeth correctly to remove the residues adhering to the surface of teeth, which can play the role of cleaning teeth and preventing caries; supplement fluorine in appropriate amount and use fluoride toothpaste, which can change the surface structure of tooth enamel and make the teeth harder; eat less sweet food and develop the good habit of rinsing mouth after meals to prevent caries.