From the initial crown discoloration to the final cavity formation, it takes one and a half to two years. The cavity is the infection of the hard tissue of the tooth, because of not carefully brushing and rinsing the mouth makes the disease-causing bacteria decompose the food residue and produce acid, the acid stays on the tooth surface for a long time, it will corrode the tooth and make the tooth demineralized, and the cavity will be formed after a long time. During this period, if there is discoloration of teeth, such as chalky plaque, uneven surface of teeth, demineralization of tooth enamel and superficial caries, or there is a small cavity, we must go to timely treatment to remove the infected hard tissues and fill the tooth surface with composite resin, the cavity can not heal by itself, but will only become bigger and bigger. The cavity can not heal by itself, but will only become bigger and bigger until it develops into pulpitis and causes severe pain.