The causes of tooth acidity when drinking cold water, including dentin exposure, gum recession, deeper cavities, etc., are as follows: First, when chewing hard food frequently, causing heavy wear on jaw surface, enamel disappears, dentin is exposed, cold stimulation can be conducted to pulp through the end of dentin tubules, and acidity occurs. Second, when the alveolar bone is heavily atrophied and the gums recede to expose the root surface, the thinner dental bone cannot tolerate the cold stimulation. When deep cavity is formed in the crown of the tooth, the bottom is close to the pulp angle, the stimulation of cool water can be transmitted to the pulp through the dentin, and cause soreness. When the incorrect brushing posture is adopted, causing wedge-shaped defect in the cervical part of the tooth, the cold stimulation can easily pass through the dentin and provoke the pulp, causing tooth pain. Fifth, when a hidden fracture occurs in the crown of the tooth, the stimulation of cold water can be transmitted to the pulp along the fracture and cause soreness.