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Dry ice is the solid form of carbon dioxide. It is used primarily as a cooling agent, but is also used in fog machines at theatres for dramatic effects. Its advantages include lower temperature than that of water ice and not leaving any residue (other than incidental frost from moisture in the atmosphere). It is useful for preserving frozen foods where mechanical cooling is unavailable. Dry ice is colorless, non-flammable, with a sour zesty odor. The density of dry ice increases with decreasing temperature and ranges between about 1.55 and 1.7 g/cm3 below 195 K (−78 °C; −109 °F). The low temperature and direct sublimation to a gas makes dry ice an effective coolant, since it is colder than water ice and leaves no residue as it changes state. Its enthalpy of sublimation is 571 kJ/kg (25.2 kJ/mol). Dry ice is non-polar, with a dipole moment of zero, so attractive intermolecular van der Waals forces operate. The composition results in low thermal and electrical conductivity. The most common use of dry ice is to preserve food, using non-cyclic refrigeration. It is frequently used to package items that must remain cold or frozen, such as ice cream or biological samples, absent the use or practicality of mechanical cooling. Dry ice is critical in the deployment of COVID-19 vaccines, which require storage at ultra-cold temperatures along their supply line. Dry ice can be used to flash-freeze food or laboratory biological samples, carbonate beverages, make ice cream, solidify oil spills and stop ice sculptures and ice walls from melting. Dry ice can be used to arrest and prevent insect activity in closed containers of grains and grain products, as it displaces oxygen, but does not alter the taste or quality of foods. For the same reason, it can prevent or retard food oils and fats from becoming rancid. When dry ice is placed in water, sublimation is accelerated, and low-sinking, dense clouds of smoke-like fog are created. This is used in fog machines, at theatres.