太阳烘烤着泥土，使其变得非常坚硬。
实际上，是角蛋白使它们变得富有弹性，并且没有角蛋白的话，它们会变得非常坚硬。
In fact, it's keratin which makes them flexible and without it they would be very rigid.
罗马人观察到，当火山灰与水混合然后冷却，火山灰变得非常坚硬，几乎不可能分解。
The Romans observed that, when volcanic ash mixes with water and then cools, it gets extremely hard and almost impossible to break up.
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