Bones

Bones are the hard parts inside your body that hold you up and give you your shape. Together, all your bones form the skeleton. Adult humans have 206 bones. Babies are born with about 300, but many of those small bones grow together as the baby grows up.

Bones are made mostly of two things: a mineral called calcium and a stretchy material called collagen. The calcium makes bones hard. The collagen keeps them from snapping like dry sticks. This mix lets bones be both strong and a little bendy. Pound for pound, bone is stronger than steel.

Your skeleton has many jobs. It holds your body up so you can stand and walk. It gives your muscles something to pull against, which lets you move. It also protects the soft parts inside you. The skull is a hard helmet around your brain. The ribs form a cage around your heart and lungs. The spine guards the bundle of nerves running down your back.

Bones are not solid all the way through. The outer layer is hard and dense. Inside, the bone looks like a sponge, full of tiny holes. This design makes bones light enough to carry around but still strong. In the middle of many bones is a soft material called bone marrow. Marrow is where your body makes new blood cells. Every second, your marrow makes about two million red blood cells.

Bones are alive. They grow when you grow, and they slowly rebuild themselves your whole life. If you break a bone, your body grows new bone tissue to fix the crack. After a few months, the broken spot can be as strong as it was before. Doctors use a hard cast to hold the bone still while it heals.

Where two bones meet, you have a joint. Some joints, like your knee and elbow, work like hinges. Your shoulder is a ball-and-socket joint, which is why your arm can swing in almost any direction. The bones in your skull, though, are locked tightly together and do not move at all.

The smallest bone in your body is the stirrup, deep inside your ear. It is about the size of a grain of rice. The longest bone is the femur, in your thigh, which can be more than a foot and a half long in a tall adult. To keep your bones strong, you need calcium from foods like milk, leafy greens, and beans, plus plenty of exercise.