There are many reasons why a small yellow spot in the mouth is hard to touch, such as ectopic sebaceous glands, dental calculus, oral ulcers and so on. 1. Ectopic sebaceous glands: sebaceous glands are ectopic and misplaced, which can cause a small yellow spot in the mouth that feels a little hard, the disease occurs in the oral cavity, lips and other locations, usually without conscious symptoms. 2. Dental calculus: the mouth up a small yellow spot feel some hard may also belong to the dental calculus, due to plaque for a long time attached to the surface of the teeth, the later will be mineralized, the formation of yellow particles. 3. Mouth ulcers: a small yellow spot in the mouth feel some hard may also belong to the ulcers, especially improper diet, mechanical trauma and other factors. There are many reasons for a small yellow spot in the mouth feel some hard, after the symptoms are recommended to go to the hospital in a timely manner to improve the examination, in accordance with the doctor’s instructions to take standardized treatment, not to private treatment, so as not to lead to adverse consequences.