Answer
Several bones in babies and children fuse together to become one bone in an adult. Adults may have differing numbers of bones due to normal variation in the development of sesamoid bones and extra bones in the skull called sutural or wormian bones.
Work Step by Step
Babies are born with several bones separated to allow for growth. A common example are the bones of the skull. The skull is several bones fused together. In a baby, these bones aren't fused yet to allow for the baby's brain to grow and develop. Over time, as the child ages, these bones fuse together. Another example is the pelvic girdle.
There's a lot of variation in adults, so sometimes people have an extra bone or are missing a bone.
