📖 科普详解
📖 Science Details
主题引入
Introduction
夏天拿出一杯冰饮料，没过多久，杯子外面就冒出一颗颗小水珠，像杯子在流汗。可玻璃杯又没有汗腺，这些水到底从哪里来的？难道饮料偷偷穿墙跑出来了吗？
In summer you take out a cup of iced drink, and before long little beads of water appear on the outside, like the cup is sweating. But a glass cup has no sweat glands, so where does this water come from? Did the drink secretly leak through the wall?
常见误解
Common Misconceptions
很多人以为水珠是杯里的饮料从缝隙里渗出来的。其实，就算杯子装的是密封好的冰水，外壁也一样会结水珠。真正的水源在空气里：空气中本来就藏着看不见的水蒸气。
Many think the beads are the drink seeping out through cracks. Actually, even a sealed cup of iced water gets beads on the outside. The real source is in the air: the air already hides invisible water vapor.
核心原理
Core Principle
空气像一块看不见的湿毛巾，里面带着水蒸气。平时水蒸气分散在空气中，看不见也摸不着。冰饮料把杯壁变得很冷，杯壁附近的空气也跟着降温。
冷空气能容纳的水蒸气比较少。当地面附近的空气被冷杯壁降到一个叫“露点”的温度时，它装不下那么多水蒸气了，多出来的水蒸气就会抱团变成液态小水滴，贴在杯子外面。这一步叫凝结。
The air is like an invisible damp towel carrying water vapor. Usually the vapor is scattered in the air, unseen and unfelt. The iced drink makes the cup wall very cold, and the air near the wall cools too. Cold air can hold less water vapor. When the air near the ground is cooled by the cold wall to a temperature called the dew point, it cannot hold so much vapor, and the extra vapor clumps into liquid droplets that stick to the cup outside. This step is called condensation.
生活应用
Real-World Application
冬天从室外进到温暖房间，眼镜容易起雾；洗热水澡后，浴室镜子会蒙上一层雾；清晨草叶上的露珠，也都是空气里的水蒸气遇到较冷的表面凝结出来的。冰杯上的水珠和它们是同一招。
给冰饮料套上杯套或用保温杯，可以减少杯壁把外面空气冷却得太厉害，所以外壁的水珠也会少一些。
In winter, coming from outside into a warm room, glasses fog up; after a hot shower, the bathroom mirror fogs; the dew on grass at dawn is also water vapor in the air condensing on a cooler surface. The beads on an iced cup use the same trick. Sleeving an iced drink or using a thermos reduces how much the wall cools the outside air, so there are fewer beads.
延伸知识
Extended Knowledge
天气越潮湿，空气里水蒸气越多，冰杯越容易结出一大片水珠。水珠多到互相合并，就会顺着杯壁往下流；这不是杯子漏水，而是空气里的水被“挤”成了小水滴。
The more humid the weather, the more water vapor in the air, and the more a cold cup beads up. When beads merge they run down the wall; this is not a leaking cup but air squeezed into droplets.
一句话总结
One-Line Takeaway
冰杯让附近空气变冷，空气里的水蒸气装不下了，就凝结成杯子外面的水珠。
An iced cup cools the nearby air, which can no longer hold its water vapor, so the vapor condenses into beads on the outside of the cup.