Tooth acidity is caused by the following reasons: 1, dentin hypersensitivity, refers to the tooth surface enamel, through the patient chewing hard objects gradually wear out, exposing the inner dentin tubules, occurring cold, hot, sour, sweet sensitivity. 2, wedge-shaped defects, refers to the occurrence of enamel defects in the buccal side of the tooth neck, usually due to the patient brushing horizontally, and this disease is good to occur in the corner area of the teeth . 3.The tooth itself exists caries. If the caries has developed to the moderate level, it is likely that the teeth will become sour when eating cold, hot, sour or sweet.