The wear and tear of teeth can lead to sensitivity and irritation pains such as soreness. There is also a very common possibility of irritation pain, which is often called worm’s tooth, the scientific name of which is “tooth decay”. Tooth decay is not caused by worms, but is a kind of chronic damage caused by the multifactorial action of bacteria, which leads to the change of color, shape and texture of hard tissues of teeth. After tooth decay, the texture is soft and soft, loses its luster, and the color is chalky, yellow or even black. With the development of the disease, the hard tissues of the teeth in the lesion area will be gradually lost, forming depressions and cavities, and even most of the crowns of the teeth will be missing, forming residual roots and affecting the chewing function. With the different stages of caries process, different nature of pain expression will appear. At the first stage of formation, there may be no special feeling, and at a certain stage of development, we can see the discoloration of the decayed part and the pain of hot and cold stimulation, and with the development of the lesion, the duration of the pain caused by each stimulation will be longer and longer, until the spontaneous pain appears, that is, the continuous pain appears even without stimulation, and even sleep at night because of the continuous pain and cannot sleep, and feel that it is not clear which tooth is in pain. This is what we call spontaneous pain. The appearance of continuous spontaneous pain is that the condition is not only the simple caries, but also the pulpitis formed on the basis of the progress of caries, which is often said to be the inflammation of the tooth nerve. Such development from no pain to severe pain actually verifies the old saying that “a small hole not patched becomes a big hole, and the dike of a thousand miles collapses in an anthill”. Severe pulpitis is partly caused by the small hole in the tooth – “tooth decay”. So how does tooth decay form? Generally speaking, there are mainly four factors: (1) microorganism and plaque factor, caries-causing microorganism mainly includes Streptococcus pyogenes, Actinomyces and other acid-producing and acid-resistant bacteria. The plaque is a film-like material adhering to the surface of teeth, and the bacteria grow and metabolize in the plaque and produce acid by fermenting food. The metabolic products of bacteria destroy the teeth and periodontal tissues, and the acid can demineralize the inorganic materials and dissolve the organic materials of teeth, forming cavities and becoming caries. (2) Food factor, food is the material basis of bacterial caries, and sugar (carbohydrate) is the most important food to induce caries. Especially, sucrose and refined sugar produce acid through bacterial metabolism, and then cause tooth destruction. It is reported that there is a positive correlation between sugar food and bacteria adhesion on teeth, maintaining growth and caries-causing ability. (3) Oral environment factor, the oral environment which has obvious influence on the occurrence and development of caries involves two major aspects: teeth and saliva. The dental factors include the tissue structure of the teeth, the external shape of the crown and the alignment of the teeth. The teeth with poor development and calcification are susceptible to caries. The grooves and sockets on the tooth surface are easy to retain food residue, which is a suitable environment for bacterial parasitism, growth and reproduction, and is also the starting site of caries. Overlapped or crowded teeth are not easy to self-clean during chewing, and their food residue retention area is often a good area for caries. The quality and quantity of saliva, buffering ability and antibacterial ability are all related to the occurrence of caries, and less saliva is easy to make caries develop rapidly. (4) Time factor, caries is a chronic destructive disease that occurs in teeth, and it takes enough time for bacteria to cause caries. From the formation of dental plaque to the occurrence of caries usually takes 6-12 months, a healthy tooth to the discovery of obvious caries (rampant caries exception) usually takes 1~2 years. Therefore, regular oral examination is very meaningful for early detection and early treatment of caries. In addition, the occurrence and development of caries is also related to gender, race, family, heredity, living environment and general health condition. Because the occurrence of caries is related to the plaque and food retention and long-term adhesion to the tooth surface. Therefore, the most common site of caries is the place where the plaque is not easy to be removed, which is clinically called the retention area. The common retention areas are the sulcus of the occlusal surface, the contact area of the adjacent surface of two teeth and the cervical part of the teeth. The sulcus of teeth is the weak link left behind in the process of development and mineralization, and it is also the primary site of caries, and there are large individual differences in its shape, width, depth, thickness and mineralization of enamel at the bottom. The wide and shallow and slightly open sulcus is easier to be cleaned, and the enamel at the bottom of the sulcus is thicker, so the caries rate is relatively low; the narrow and deep sulcus is easy to have plaque and food residue and not easy to be cleaned, so it is more prone to caries. Adjacent surface of teeth is the site of caries which is second only to sulcus. Generally, it is caused by the contact of the adjacent surface is not tight or the interdental gingival papilla is atrophied, which leads to food embedding and plaque retention and caries. Often, two adjacent teeth get caries at the same time. Other teeth, such as tooth misalignment and tooth twisting, can also lead to food embedding and caries. The cervical part of the tooth is the part where the enamel meets the bone of the root, which is not only easy to retain the plaque and food but also the weak link of the dental tissue, especially when there is no contact between the enamel and the bone and the dentin is exposed, it is easy to get caries. When the gingival atrophy causes the root to be exposed, the root surface caries is easy to occur, and this kind of caries is mostly seen in the elderly and periodontal disease patients. It should be noted that the early caries on the smooth surface of the tooth – plaque – can often disappear after a period of time by quiescence or remineralization, which is mainly related to easy cleaning. Knowing these, we can target to do early prevention, early detection and early treatment of tooth decay. That is to brush teeth effectively on weekdays; regular oral examination; early remedy to nip the bud.