How Bones Grow
When you were a baby, you had tiny hands, tiny feet, and tiny everything! Slowly, as you grew older, everything became a bit bigger, including your bones.

A baby's body has about 300 "soft" bones at birth. These eventually fuse (grow together) to form the 206 bones that adults have. Some of a baby's bones are made of a special material called cartilage (say: car-til-ij). This cartilage is soft and flexible. During childhood, as you are growing, the cartilage grows and is slowly replaced by bone, with help from calcium.

By the time you are 25, this process will be complete. After this happens, there can be no more growth - the bones are as big as they will ever be. All of these bones make up a skeleton that is both very strong and very light.

Your Spine
Your spine is one part of the skeleton that's easy to check out: reach around to the center of your back and you'll feel its bumps under your fingers.

The spine lets you twist and bend, and it holds your body upright. It also protects the spinal cord, a large bundle of nerves that sends information from your brain to the rest of your body. The spine is special because it isn't made of one or even two bones: it's made of 33 bones in all! These bones are called vertebrae (say: vur-tuh-bray), and each one is shaped like a ring.

There are five types of vertebrae in the spine, and each does a different kind of job:

• The first seven vertebrae at the top are called the cervical (say: sir-vih-kul) vertebrae. These bones are in the back of your neck, just below your brain, and they support your head and neck. Your head is pretty heavy, so it's lucky to have help from the cervical vertebrae!
• Below the cervical vertebrae are the thoracic (say: thuh-rah-sick) vertebrae, and there are 12 in all. These guys anchor your ribs in place.
• Below the thoracic vertebrae are five lumbar (say: lum-bar) vertebrae.
• Beneath the lumbar vertebrae is the sacrum (say: say-krum), which is made up of five vertebrae that are joined together.
• Finally, all the way at the bottom of the spine is the coccyx (say: cok-sicks), which is made of four fused vertebrae. The bottom sections of the spine are important when it comes to bearing weight and giving you a good center of gravity. So when you pick up a heavy backpack, the lumbar vertebrae, sacrum, and coccyx give you the power. When you dance, skip, and even walk, these parts help keep you balanced.

In between each vertebra (the name for just one vertebrae) are small disks made of cartilage. These disks keep the vertebrae from rubbing against one another, and they also act as your spine's natural shock absorbers. When you jump in the air, or twist while slamming a dunk, the disks give your vertebrae the cushioning they need.