Human babies are born with about 300 bones, but adults only have 206.

The process of bones joining together and hardening is called ossification. It begins before birth and usually finishes when a person is in their mid-20s. A baby's skull has several separate plates with soft spots between them. These gaps let the head squeeze through the birth canal and give the brain room to grow fast.The sacrum at the base of the spine starts as five separate pieces in a child. By age 25, these pieces fuse into one solid bone. The pelvis also starts as three different bones called the ilium, ischium, and pubis before joining together. More than half of the bones in an adult body are located in the hands and feet.The smallest bone in the human body is the stapes, located in the middle ear. It is only 3 millimeters long. Bones are living tissues that constantly replace themselves. Every year, your body replaces about 10% of your skeleton with new bone cells.If bones did not fuse, the human body would be too floppy to stand or walk upright. Cartilage is soft and allows a child to grow quickly. Hard bone provides the strong support needed for an adult's weight. Most adults end up with 206 bones, though some people have extra ribs or small toe bones.