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Here’s something we tend to take for granted: drop a few ice cubes into a glass of water at room temperature, and in a few minutes the water will cool. How does this work?
有件事情我们总觉得理所当然：室温下把一些冰块放入水中，过几分钟水就会变凉。但这到底是什么原理呢？

Heat is a measure of molecular¹ activity: the faster a thing’s molecules² move, the more heat that substance contains. And because everything is made of molecules, and molecules are in constant motion, everything contains at least some heat.
热量是衡量分子运动的一个指标：一个物体分子的移动越快，物体所包含的热量就越多。由于一切物体都由分子组成且分子总在不断地运动，所以每个物体或多或少都包含着一些热量。

As we know, ice is colder than room temperature water. Because ice molecules move slowly and cluster tightly together, they produce a relatively³ low amount of heat. When ice is placed in a glass of water whose molecules are moving at a greater rate, the ice begins to absorb energy, because heat always travels from regions of relative warmth to colder areas in order to equalize temperatures. In other words, ice absorbs heat from the water.
我们都知道，冰的温度要比温水低。由于冰分子移动缓慢且相互连结得十分紧密，因此它们产生的热量相对较少。如果把冰块放入一杯分子移动较快的水中，冰块就会开始吸收热量，这是由于热量总是会从高温地带转移到低温地带，从而使温度均衡。也就是说，冰块从水中吸收热量。

As the water molecules lose energy, they begin to slow down, and consequently to cool. So, it’s kind of the opposite of what we might think: when we put ice in water, the ice doesn’t give its cold to the water, it takes heat from the water.
水分子在失去热量后运动减慢，继而水温降低。这可能跟我们所想的恰恰相反：当我们把冰放入水中，冰不会把自己的低温传给水，而是从水中吸收热量。

The cooling process actually happens in stages. Because ice floats, it cools the water near the top of the glass first. As the top layer of water cools, its density⁴ increases, causing it to sink to the bottom. The lower layer, less dense⁵ and therefore more buoyant, floats to the top and is cooled in turn.
这个冷却过程实际上是分阶段进行的。由于冰块会浮在水面上，所以最上层的水会先变凉。上层的水冷却后，它的密度会增大，上层的水会往下沉。而下层的水由于密度较小于是会上浮，进而又被冷却，依次循环下去。