hardening in boiling water

例句：

1. 1.The metal has hardened after being exposed to the sun for a long time.
   金属在长时间暴露在阳光下后变硬了。
2. 2.Hardening is a process that occurs when a material is subjected to heat and pressure.
   硬化是一种过程，当材料受到热和压力时会发生。
3. 3. The car's engine has hardened over time due to regular use.
   这辆汽车的发动机因为经常使用而变硬了。