A baby is born with more bones than an adult because some later fuse.

The longer version

Newborn skeletons contain many separate elements that later join together. A large share of the difference comes from areas where bone starts as cartilage or as several pieces separated by gaps; over the growing years those pieces ossify and the gaps close, producing a single bone in the adult.

The skull and the pelvis are the clearest examples, since both begin as multiple parts. Fusion is also practical: a baby's skull needs some give during birth and room to expand afterwards. By early adulthood the count has settled, and the remaining joints between bones are the ones that allow movement. Growth plates at the ends of long bones behave the same way, closing once lengthening stops.

Where this entry sits

These facts look at how the human body works in ordinary moments, from breathing and blinking to balance and sleep. They focus on anatomy, senses, and small reactions that happen constantly without any conscious effort. The full list sits on the Human Body page, where every entry is listed in order.