Shark’s tooth is correctly called milk tooth stagnation, milk tooth stagnation is not a genetic disease, its cause may be ectopic eruption of permanent teeth, periapical lesions of milk teeth, etc., details are as follows. 1. Ectopic eruption of permanent teeth: the direction of eruption of permanent teeth is abnormal, resulting in the root of the milk teeth is not normally absorbed, and can not fall off normally, the milk teeth and ectopic eruption of permanent teeth form a “double row of teeth”, similar to the shark’s teeth. 2. Periapical lesions of milk teeth: Periapical lesions of milk teeth can destroy the alveolar bone and lead to early eruption of permanent teeth, at this time, milk teeth can not fall off, and early eruption of permanent teeth to form a “double row of teeth”. There may be other reasons for the retention of milk teeth, it is recommended to go to the hospital in a timely manner, to clarify the cause of the disease and then targeted treatment.