This is a great question – all of our bones are made up of a few different parts – usually three. The main one is the diaphysis (this is the middle bit) and then two epiphyses (two end bits). If you imagine your upper arm bone (the humerus) it has a top end (near your shoulder – that’s one epiphysis) and middle bit (the long straight part – that’s the diaphysis) and then bottome end (near your elbow – that’s one epiphysis). When we are still growing, inbetween the ends and the middle there are squidgy bits called ‘growth plates’. These are specially designed to only add new layers of bones to the ends of the middle bone. This keeps it growing longer in both directions, from the middle, until it is long enough. Then, the end bits and the middle bit fuse together and it stops. This means that all of the bones grow from the middle out, making sure they grow in the correct directions. Neat, isn’t it?