A round hard thing sliding under the skin is considered to be a sebaceous cyst, enlarged lymph node or subcutaneous lipoma. 1. Sebaceous cysts: If it is a single hard lump under the skin, it may be a sebaceous cyst, which may exist for many years and have no self-conscious symptoms, but some of the cysts are also susceptible to infections, pus and ulceration, which are usually treated by surgical excision. 2. Enlarged lymph nodes: If there are round and hard nodules under the skin in the armpit, behind the ear or under the jaw, it is considered that the lymph nodes may be enlarged, which is mainly due to bacterial infection, reactive hyperplasia and other factors, and it is necessary to go to the hospital to clarify the cause of the disease and to take targeted therapeutic measures. 3. Subcutaneous lipoma: Subcutaneous lipoma is a kind of benign subcutaneous tumor composed of mature fat cells, which is usually manifested as a confined mass with clear boundary, soft texture and movable, so there is a round hard thing sliding under the skin, which may be subcutaneous lipoma. Since there are many factors that cause a round hard thing sliding under the skin, it is recommended to go to the hospital for examination and diagnosis, to clarify the cause of the disease as soon as possible, and to carry out active targeted treatment, so as to avoid misunderstanding of the condition and causing symptoms to aggravate and cause adverse effects.