A tooth root that becomes sore when touched with a fingernail may have a wedge-shaped defect in the neck of the tooth, usually caused by improper brushing, usually due to lateral brushing. Usually the correct way to brush teeth should be done with the Bartholin method or the circular method, not with the lateral brushing method, which can easily cause gum defects and tooth defects. When the dentin layer is exposed when the tooth is defective, it is easy to have dentin sensitivity, especially probing sensitivity, mechanical stimulation to produce sensitive symptoms; and in serious cases, it is easy to have hot and cold irritation pain, especially easy to have night pain, then root canal treatment should be done. And if the nail just appears to be sour when touched, resin filling or glass ion filling can be carried out, and it is recommended to apply fluoride to promote its remineralization and prevent secondary caries.