Fish skin consists of two layers, a thin outer epidermis and a thicker underlying dermis. A variety of gland cells in the epidermis secrete protective chemicals, the most evident of which is mucus, a proteinaceous substance much like egg white. The skin is often richer than the flesh, averaging 5–10% fat. The thick dermis layer of the skin is especially rich in connective tissue. It’s generally about one-third collagen by weight, and therefore can contribute much more thickening gelatin to stocks and stews than the fish’s flesh (0.3–3% collagen) or bones. Moist heating will turn the skin into a slick gelatinous sheet, while frying or grilling enough to desiccate it will make it crisp.