Simple fact no matter what it is ,or how big it is, Little things is what makes it possible. You take away the little things and there no big thing. So little things do matter Most of the human body is made up of water, H2O, with cells consisting of 65-90% water by weight. Therefore, it isn't surprising that most of human body's mass is oxygen. Carbon, the basic unit for organic molecules, comes in second. 99% of the mass of the human body is made up of just 6 elements: oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus.