Bone contains a substance which long cooking changes into a jellylike mass called gelatine. In the center of the bone there is a fatty substance called marrow. This fat in the bone and that in and around the muscles liquefies in making soup stock. In cooling, the fat rises to the top, hardens, excludes the air from the stock, and thus prevents it from spoiling readily. Hence, in soup-making, it is of advantage to use both the fat and the bone with the lean meat. The fat, however, should be removed carefully from the stock before using.