Height is determined by length of the long bones and skeleton. Arms and legs have special areas across them (below the knees and wrists) called “growth plates” made from flexible cartilage. The cartilage is turned into hard bone in these areas by bone cells, called bone matrix. Matrix itself is made of a material called “hydroxyapatite” and it is like a chalky ceramic. Matrix is added a layer at a time onto the end of the plates, gradually making the bone longer.