Bone, although strong, is a constantly changing tissue that has several functions. Bones serve as rigid structures to the body and as shields to protect delicate internal organs. They provide housing for the bone marrow, where the blood cells are formed. Bones also maintain the body's reservoir of calcium. In children, some bones have areas called growth plates. Bones lengthen in these areas until the child reaches full height, at which time the growth plates close. Thereafter, bones grow in thickness rather than in length, based on the body's need for additional bone strength in certain areas.