Do you know the radical called renga?
In kanji, it is written as 連火 and is also called rekka (列火).  Fire is symbolized by four marks 灬 when written on the under part of a kanji.  Today, we will introduce kanji containing the radical renga.

First of all, 煮 “boil”.
This kanji comes from symbolizing “burning wood over a stove,” and also has the meaning of “making a fire by centering heat.” It is read as niru, and means to pour water over food and place it over fire.

Next is 蒸 ”steam.”
Just as the kanji compounds 水蒸気 “vapor” and 蒸発 “evaporate” indicate, it represents small water droplets rising. When this kind of “vapor” is applied to cooking food, it is called “steamed.”  蒸 which is formed by kusakanmuri and 烝 (meaning hot air rising) originally came from plants growing abundantly up and up, and evolved to have the meaning to “steam”.

“Boil” and “steam” both mean to give heat by fire, but the character  熱 also has renga.
The 埶 written on the top part is to represent pronunciation, therefore the meaning for 熱 is concentrated in 火. This kanji represents “fire burning and it is hot”, and eventually came to be used to express heat without any relation to fire.  熱中 is the state of being absorbed or heated on one thing, and is an example of this.

Fire is an absolute necessity for humans, therefore there are many kanji that came about containing hihen and renga.