牙结石—如果菌斑呆的时间足够长，它就开始矿化变硬形成牙石或牙垢，因为菌斑从唾液中吸收了钙，磷和其他矿物质。

Calculus - If left alone long enough, plaque begins to mineralize and harden into calculus or tartar because the plaque absorbs calcium, phosphorus and other minerals from saliva.

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由于从小在海边长大，我知道在退潮后的某段时间，沙子由于潮湿会变硬，所以我也经常在这个时候在沙滩上赤脚跑。

But, having grown up by the ocean, I know that for a certain time after the tide has gone out, the sand is solid from the moisture and is more suitable for running, so I run barefoot there often too.

youdao

雪下落时，会在地面上层层累积。只要时间足够，这些雪层就会变硬并相互结合。

As snow falls, it builds up layer upon layer. With enough time, the layers will harden and bond to one another.

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