Bones have a soft part called cartilage where they are connected to the bone next to them. When a bone grows, this cartilage grows first. The cartilage becomes hard in the order in which it grows first, and then becomes bone, and the bone grows little by little. When the cartilage stops growing, the bone does not grow any longer. So why does cartilage stop growing in adulthood? The exact answer is still not well understood. This is another mystery of the human body.