When a child changes 20 teeth, at the age of 5-6 years, the milk incisors begin to loosen and are gradually replaced by permanent incisors, and at the same time, in the distal middle of the milk molar, the first permanent molar will begin to grow.Between the ages of 6-12 years, during the period of milk and permanent tooth replacement, the 20 milk teeth will be successively replaced by the inherited permanent teeth.At the age of 12 years, the second permanent molar may erupt in the distal middle of the jawbone, and at this time, there are a total of 28 permanent teeth in the mouth, and the establishment of the permanent dentition (dental cementum). Since the first and second permanent molars erupt directly in the oral cavity and are not replaced by milk teeth, the other 20 permanent teeth, excluding the four permanent molars, grow after the milk teeth are lost. For the newly erupted permanent teeth, oral hygiene and cleaning need to be strengthened to prevent the formation of caries and prolong the life of the teeth.