The main ingredient of meat is protein, and protein is organic matter. When heated, it will expand when heated and contract when cooled, resulting in looseness. The main component of bone compound is calcium. These compounds are inorganic and will not expand when heated and contract when cooled, so they will separate. The skin is mainly fat. The function of fat is different from that of protein. Maybe you heat it too fast, and the effect is similar. Try heating it slowly. In addition, it may also be caused by uneven heating. Ha ha. I have told you everything I know.