The whole body is covered with skin.The surface of the skin is dead(like the hair and the nails),but it is made from a living layer just under the surface.The outside dead layers wear off all the time,with new ones replacing them,and the body forms a totally new skin every three weeks. The skin protects the body from the outside world and it passes information from the outside.It helps to keep water in the body and to keep the body at the same temperature.Becteria can't get in through the skin unless it is damaged. If the weather is cold,the blood vessels in the skin squeeze down,or contract ,so that as little blood as possible comes near the surface.This means that less warmth is lost to the outside.This makes the skin look pale,or even "blue".At the same time,the hairs on the skin are pulled upright by tiny muscles. When it is hot,the blood vessels expand ,or get wider again,to allow plenty of blood to run near the surface of the skin.This helps the body to lose heat,and the skin may look red.At the same time,the sweat glands produce more sweat,which cools the skin. If your skin has ever been numb ,you will realize how important the feeling from the skin is.Nerves in the skin are sensitive to heat and cold,sharpness,hardness and softness,wetness and dryness and pain.If one is not able to feel these things,which can act as warnings,he is likely to be burned,cut or hurt in other ways. The colour of the skin depends on special colour cells .These are just below the top layers of the skin.They are needed as protection from some harmful ray of the sun.The stronger the sun is,the more important it is to have a dark skin.People with pale skins may "tan" in the sun.This happens when the body produces extra colour cells in the skin for protection.In the sun,the skin produces a vitamin(vitamin D)which is needed for strong bones. The colour of the skin is determined by  _  .
the rays of the strong sun colour cells top layers of the skin vitamin D