The bones in our body form our skeleton. They help to support our body and protect important organs. Bones also store nutrients and minerals, and they are places where blood cells are made. Humans have more than 200 bones in their bodies-all of different shapes and sizes. Like other parts of our body bones keep changing all the time. They are alive and grow, especially when you are young. The outer surface of a bone is called the periosteum. It is a very thin membrane that has nerves and blood vessels in it. They carry nutrients to the bones. The compact bone is the smooth and very hard part of the bone. It is the part you see when you look at a skeleton. Spongy bone is lighter than compact bone but it is still very strong. It looks like a sponge or honeycomb with a lot of spaces in between. The inside parts of a bone are hollow. They are filled with a jelly called bone marrow. In adults the long bones of the legs and arms are filled with yellow marrow. The ends have red marrow. It is the place where billions of new blood cells are produced every day. When a child is born it has about 300 "soft" bones . As it grows up some of these bones grow together to form the 206 bones that a normal grown-up has. During childhood bones grow with the help of calcium. By the time you are about 20, bones stop growing. The spine is a special part of our skeleton. It has 26 small bones that look like rings and are linked together. These rings are called vertebrae. The spine lets you twist and bend your body and it holds your body upright. It also supports your head so that it can't fall down. The spine protects the spinal cords which are the nerves that send information to the rest of your body. Between each ring there are small soft discs. They keep the vertebrae from rubbing against each other and act as a pillow, so when you jump into the air and come to the ground again it doesn't hurt. Ribs are a cage of bones that protect the most important organs: your heart, lungs, liver and others. You can feel your ribs by touching your chest. Most people have twelve pairs of ribs that look the same on the right and left side. They are attached to the spine in the back. In the centre of your chest there is a strong bone called the sternum. It keeps your ribs in place, so that they don't fall apart. The bones in your head make up your skull. They protect your brain. Some bones in your skull are fixed, so that you can't move them. Others, like your jawbone can be moved. It opens and closes your mouth when you eat, talk or chew food. Babies are born with spaces between the bones in their skull. As a baby grows the spaces disappear and the bones grow together. Your arms and hands make up a total of about 54 bones. They let you write, pick up things or throw a ball. Each arm is attached to a shoulder blade. There are two long bones in your arm which are connected through your elbow. These bones are wider at the ends and thinner in the middle. The wrist connects the lower part of your arm with your hand. It is very flexible so you can twist it and turn it around in many directions. The hand is made up of small separate bones. Each finger has three bones, only your thumb has two. A leg is made up of three bones. They are very large and strong and help support the weight of your body. They are connected to a group of bones called the pelvis, which supports the upper part of your body. The longest bone of our body is the femur. It runs from the pelvis to the knee. The knee itself is protected by the kneecap. The ankle connects the lower part of your leg with your foot. The bones in your feet help you stand and balance your body. A joint is a place that holds two bones together. Fixed joints don't move at all. Some of them are in your skull and in other parts of your body. Moving joints allow you to twist, bend and move different parts of your body. Some of them let you move in only one direction, others allow you to move freely in many directions. Joints have fluids in them, so they can work more easily and don't hurt. bones are xrayedLike all parts of the body our bones can also suffer from certain diseases. Bones need calcium to make them hard. If your body does not get enough vitamin D bones don't have enough calcium, they become soft and curved. People suffer from osteoporosis when they have weak bones. Bones can also break. In babies and young children broken bones grow together quickly, but when people get older it takes longer for bones to be repaired. In elderly people bones heal very slowly-sometimes not at all. Joints also attract diseases when a person gets older. Arthritis is an inflammation of the joint that hurts and sometimes makes it impossible for you to move.