That one’s simple. Some bones (such as the bones in the skull) fuse together over time. The skull has to be a little flexible at birth because, hey, you’re trying to squeeze a bowling ball through an opening the size of a paper towel tube. (Or trying to squeeze a microwave through an opening the size of a lemon, as my neighbor’s wife once said.) If those bones were fused at birth, all babies would have to be delivered by C-Section, because the head is too big to pass through the pelvic canal.At birth, the skull is made out of 44 separate bony elements. (EDIT: That includes the jaw and facial bones.) By the time we’re adults, there are only 22, because some of the bones fuse together. The leg and arm bones also are unfused at birth, to allow for continued growth and to reduce injury when a baby falls. For example, babies are born with kneecaps that are mostly just cartilage and do not become one fully formed bone until the age of three.