The high alcohol level in flex fuel pulls the water into the fuel and holds it tightly. Pure alcohol and water will mix to form a clear uniform solution in all concentrations. North American fuels contain only about 10% alcohol, so most of the fuel is hydrocarbon. A small amount of water will mix clearly into the alcohol and be held within the fuel, but as the water percentage increases, the alcohol-water mixture becomes less soluble in the fuel and forms a separate layer.