水为什么是火的克星？
水能够灭火，是因为具有两种特性：
一是水碰到炙热 (zhì rè)的物体就会变成水蒸气，带走物体上的热能，降低温度；
二是水遇到热后，产生的水蒸气会占据相当大的空间，包围在燃烧的物体外，隔绝火和空气的接触。一旦没有空气中的氧帮助燃烧，火就会熄灭。
懂了吗？
Translate to aid those who do not know Chinese
Why is water a nemesis to fire?
For fire to start, we have to know that fire requires a certain temperature for oxidation to occur, resulting in proper fire with flames and all.
Water aids in putting out the fire because of two characteristics:
Firstly, when water is in contact with hot objects, it will evaporate to become steam, taking the heat away from the object thus bringing down the temperature, making it more difficult for the fire to burn.
Secondly, when water evaporates and becomes steam, it occupies a broader surface area, surrounding the area or object that is engulfed in fire. This creates a barrier where the air, especially oxygen, does not reach the fire easily, causing the fire to subside.
Source: Quoted from an educational book.
