hardens
v.
（使）变硬( harden的第三人称单数 );
（使）坚固;
（使）硬化;
（使）变得坚强;
例句:
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Steatite hardens when it is fired in a kiln.
来自BBC
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Mould the mixture into shape while hot, before it hardens.
在混合物还没有变硬之前，趁热将它塑成形。
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Clay hardens when it becomes dry.
粘土干了的时候就变硬了.
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If public disapproval hardens, critics will be emboldened to speak out.
如果公众反对态势加深, 批评人士才会敢于全盘托出.
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They plaster up the entrance with mud, which hardens.
来自THETIMES
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Even as our bones and teeth soften, the rest of our body hardens.
来自NEWYORKER