Mammal bone is an incredibly strong substance, like fibreglass it is good at surviving a variety of stresses. However it does sometimes get damaged. When it does the body's automatic bone healing mechanisms swing into action. This callus is like an internal bandage, it is gradually replaced by new structured bone material. This final process takes time, from a few weeks to a about a year, depending on the size of the bone, the degree of damage, the location of the fracture, how much stress the regenerating bone is under and the patient's age and health.
Joints occur where different bones meet.
In some joints the bones are very strongly linked together, there is no movement in these joints and the resulting pair or group of bones act as a single bone. The skull is a good example of this, the places where the bones meet are called sutures and can often be seen as squiggly lines on the skull or pelvis. These sort of joints are called 'Synarthrodial Joints'. The joints between the various vertebrae in our back bones are called Cartiliginous joints. Like synarthrodial joints they remain pretty fixed but they do have small degree of flexibility or else we would not be able to bend our backs.