Refrigeration (冷藏) is one of the most important means of keeping food. Refrigeration makes anything cold. Scientists and engineers have discovered many means of things cold.
One way is to allow water, or another liquid, to evaporate (蒸发). Evaporation cools a liquid and the liquid cools the food.
Cold can also be produced by passing a gas through a small hole. Gases are made up of molecules (分子). Before the molecules pass through the small hole, they are close together. They are also moving quickly. Afterwards they are further apart and moving more slowly. Because the molecules are moving slowly, the gas is cool. The slower the molecules move, the cooler the gas will be. If the molecules move very slowly the gas changes into a liquid. Liquid oxygen can be made by passing it through a small hole.
It is possible to keep food cool with a liquid if ______ .