Children change all of their teeth. Generally, each person has 20 milk teeth, and all 20 teeth will be replaced once during the growth process. Adults have a total of 32 teeth, so in addition to the 20 teeth that are replaced, adults will grow 3 more teeth behind the 20 teeth. Generally, the period of tooth replacement is from 6 to 13 years old, during which the middle teeth are replaced first, and then the teeth are gradually replaced on both sides. The replacement of teeth in the lower jaw is usually earlier than in the upper jaw, so the first molar incisor in the lower jaw is often replaced, and the last tooth to be replaced is usually the second molar in the upper jaw.