HOW COME?: We need our bones; they keep us upright

Why do we need bones? asks a reader. Why do babies have more bones than adults? How many bones do middle students have? asks Joey McGloine, a student in Farmingville.It seems backward: As newborns, we have about 275 bones; by age 25, that number has dwindled to about 206. How come? Between birth and our 20s, our bones go through a kind of reorganization, crafting the skeleton that will best support us as full-grown adults. And here's a surprise: You may actually have a few more (or a few fewer) small bones than the person sitting next to you at school, or on the subway.

Just as we can't live without hearts and brains, human beings can't live without their bones. Our bony skeleton holds the body up, keeping us from collapsing into a jellylike blob. It shields organs from the pokes and prods of the outside world. The hard skull protects the big brain; the rib cage keeps the heart, lungs and liver safe.

The skeleton, a living organ, also makes blood cells, replenishing our blood. And it stores minerals like calcium and phosphorus. The skeleton is also a scaffolding that muscles can latch onto; that's how we move arms and legs, fingers and toes. Lacking bones, we couldn't write, build buildings or bike down the street.

Still, most forms of life on Earth get along just fine without our kind of skeletons. Some have hard outer shells that their muscles pull against to crawl or swim. Jellyfish use pressurized water to maintain their shape. An octopus is held together by crisscrossing bands of muscle. But all of the larger land animals, from mice to elephants to us, rely on a trusty skeleton to give form to otherwise shapeless, waterlogged bodies.

All those bones make up about 14 percent of the entire mass of your body. Its largest individual bone is the femur, the heavy bone in the thigh. The smallest are the ears' hammer, anvil and stirrup, whose movements help us hear.

As a human fetus develops, most bones start out as cartilage (the stiff material we can feel in our nose and ears). The cartilage forms a model for the hard, permanent bone that gradually takes its place.

Our newborn bone count drops while we are still infants, as once-separate bone segments meld into single bones. But as we become older kids, the number rises a bit as ankle and wrist bones develop. (Seven bones in each ankle and eight in each wrist, interlocking like a jigsaw puzzle.)

Since everybody develops at a different rate, it's impossible to know exactly how many bones you will have at, say, age 12. But as we go through puberty and reach our adult heights, some small bones, like those at the end of the spine, fuse together. By the time we're about 25 years old - the age at which scientists say growth is finally finished - we're down to about 206 fully formed bones.