Only your dentist can tell for sure whether you have a cavity. That's because cavities develop below the tooth's surface, where you can't see them. When you eat foods that contain carbohydrates (sugars and starches), these carbohydrates are eaten by the bacteria in plaque, producing acids that eat into the tooth.hat's because cavities develop below the tooth's surface, where you can't see them. When you eat foods that contain carbohydrates (sugars and starches), these carbohydrates are eaten by the bacteria in plaque, producing acids that eat into the tooth.