When a tooth is cold, hot or hard, the first thing that should be done is for a professional dentist to distinguish the cause of the pain. There are two common diseases that cause this symptom. One is called dentin hypersensitivity, which is caused by tooth wear or trauma that exposes the dentin in the mouth and causes pain when stimulated because of the amount of nerves in the dentin. In this case, we can consider the application of anti-allergic toothpaste or go to a specialized dentistry department to apply fluoride or iodine preparations to seal the dentin tubules or desensitize them. There is another kind of disease corresponding to the dental pulp, namely pulpitis, which can also be painful with cold, heat and hard stimulation. In this case, the pulp can be opened with a power system in the dentistry department and finally treated with a systematic root canal treatment.