Cells. All living things are made of cells. There are of course many different kinds of cells in a complex, multicellular organism. For example, there are muscle cells, heart cells, liver cells, skin cells, brain cells, bone cells, blood cells and nerve cells. There are also stuff found between cells that are not made of cells, but they hold cells together. The most abundant substance inside a cell is water. So all living things are actually mostly made of water. Stuff like DNA, protein, carbohydrates and fatty acids are only a tiny percentage of what a living organism is made of.