In the process of human growth and development, there are two sets of teeth, one is the milk teeth, a total of 20, of which the lower milk front teeth are the earliest to erupt about 6 months after birth, and later, with the growth of age, the other milk teeth erupt one after another, until about 2.5 years old when the 20 milk teeth are completely erupted, forming a complete row of milk teeth. At around 6 years of age, the second set of human teeth begins to erupt, which are usually called permanent teeth, until around 13 years of age, when all the baby teeth are replaced by permanent teeth. The average person has only 28 permanent teeth, but some people have a third set of molars, commonly known as wisdom teeth. The number of wisdom teeth varies from person to person, some people have 1, some people have all 4. Therefore, it is normal to have 28-32 permanent teeth. The baby teeth are called from front to back: central incisor, lateral incisor, cuspid, first molar, and second molar. The permanent teeth are called: central incisor, lateral incisor, cuspids, first premolar, second premolar, first molar, second molar, and third molar, respectively, from front to back. All teeth are symmetrical with the midline, upper, lower, left and right.