A small piece of black inside the mouth may be caused by food debris, petechiae or dental caries and other diseases. 1. Food debris: If personal hygiene habits are poor, no rinsing and brushing habits after eating, resulting in food debris left in the mouth, there will be a small piece of black inside the mouth. 2. Petechiae: If you eat too fast and bite your tongue, if the local mucosa is not broken, the blood will pool locally and become petechiae, which will also lead to a small piece of black inside the mouth. 3. Dental caries: If there is a small piece of black on the teeth, consider dental caries. Dental caries is caused by bacterial infection and other factors leading to the destruction of the hard tissue of the dentin, the cause of the disease is also related to dietary factors, long-term intake of sugary foods, can also lead to the occurrence of dental caries. There are more reasons why there is a small piece of black inside the mouth, it is recommended to consult a doctor for examination and clear diagnosis.