They're hard on the outside to support our muscles and protect important organs. But bones are spongy inside. The centre contains bone marrow.
Bone marrow is important because it's the factory that makes most of our blood cells.
If something goes wrong with our bone marrow we get sick. For example, if bone marrow makes too many cells we get leukemia, a type of cancer. Sometimes children with leukemia need to receive new bone marrow to make them better. This is called a bone marrow transplant. There must be a good match between the bone marrow of the donor and patient for the transplant to work.