Lipids are a wide-ranging group of organic compounds found in all living organisms, including human beings. Lipids are the body's reserve supply of energy. The word lipid comes from the Greek word "lipos", which means "fat". Cells make lipids in the human body and, along with carbohydrates and proteins, are the components of all life. The major classes of lipids in humans are acids, glycerol-derived lipids (fats and oils), and steroids. Unlike other organic compounds, lipids are soluble only in alcohol, ether, and other organic substances, but not water. The solubility factor is determined by the chemical structure of the lipid and it's compatibility with the chemical structure of the substance in which it is placed. Lipids tend to be less dense that water, so not only do they not mix with water, the lipid layer will rise to the top, while the water layer will be on bottom.