Mammalian skin care often contains hairs, which in sufficient density is called fur. The hair mainly serves to augment the insulation the skin provides, but can too answer as a secondary sexual characteristic or as camouflage. On some animals, the skin is very hard and thick, and can be processed to create leather. Reptiles and fish have hard protective scales on their skin for protection, and birds have laborious plumage, all made of tough -keratins. Amphibian skin is not a strong barrier to passage of chemicals and is often subject to osmosis. A frog sitting in an anesthetic solution could quickly go to sleep. The pelt is often known as the largest organ of the human body. This applies to exterior superficies, as it covers the body, appearing to have the largest surface area of total the organs. Moreover, it applies to weight, as it weighs more than any single internal organ, accounting for about 15 percent of material substance weight. as being the average adult human, the skin has a surface area of between 1.5-2.0 square meters (16.1-21.5 sq ft.), most of it is between 2-3 mm (0.10 inch) thick. The average square inch (6.5 cm?) of skin holds 650 sweat glands, 20 blood vessels, 60,000 melanocytes, and more than a thousand nerve endings.