THE VISCOSITY OF A LIQUID CONSISTING OF ONLY ONE TYPE OF MOLECULE will increase when the liquid is cooled. If the liquid is cooled sufficiently, the viscosity will increase until the liquid solidifies (freezes). However, most petroleum products are composed of mixtures of many different types of molecules, each of which has its own freezing point. When these liquids are cooled to low temperatures, unanticipated results can occur. The diesel fuel for cars and trucks can form crystals taht will plug the fuel filter while the vehicle is being driven, causing the vehicle to stop. Heating oil can be loaded into a delivery vehicle, but at very low temperature the oil may not flow through the delivery hose to the business or home receiving the oil. Automotive engine products in conventional quart, liter, or gallon, containers may, when left outside the filling station in the winter, solidify so they will not pour out of the container. Some petroleum products will form a “gel” structure at a certain low temperatur. However, the same material, when cooled rapidly from a warm water to a still lower temperature and through a critical temperature range, will flow in a normal manner.