There are black lines on the big teeth but no pain because the teeth have caries, but have not decayed to the nerve, so there is no obvious pain symptoms. 1. The occlusal surfaces of the large teeth (mainly the molars) have a large number of grooves, whose main function is to be used for food grinding and overflow; however, when the grooves of the teeth are deeper and there is no good oral cleaning, it will lead to the accumulation of food in the depths of the grooves, which interacts with the bacteria and causes the teeth to develop black grooves of caries. 2. Fissure caries usually has no obvious pain symptoms when shallow or medium caries, the clinical manifestation is mainly black strip caries in the fossa; but if the caries filling and repairing is not carried out in time, resulting in aggravation of the symptoms, that is, there will be hot, cold and pain and other symptoms; therefore, it is recommended that fissure caries be treated in time by the doctor for the symptomatic treatment.