Lean meat is made up of three basic materials: it’s about 75% water, 20% protein, and 3% fat. These materials are woven into three kinds of tissue. The main tissue is the mass of muscle cells, the long fibers that cause movement when they contract and relax. Surrounding the muscle fibers is the connective tissue, a kind of living glue that harnesses the fibers together and to the bones that they move. And interspersed among the fibers and connective tissue are groups of fat cells, which store fat as a source of energy for the muscle fibers. The qualities of meat—its texture, color, and flavor—are determined to a large extent by the arrangement and relative proportions of the muscle fibers, connective tissue, and fat tissue.