字面：熱從外面來
這一組直接用字面畫面：身體或機器自己不產熱，所以溫度得靠環境給。
adjective · B1
Used about animals such as snakes, lizards, fish and frogs: having a body temperature that goes up and down with the temperature of the air, the ground or the water around them.
變溫的、冷血的（動物）
核心畫面
One way to hold this word: blood that never warms itself. Whatever it belongs to has no fire of its own inside. A snake on a cool morning cannot move until the sun lends it heat; an engine will not run smoothly until it is warmed. English then lends the same picture to people. If the blood stays cold while something cruel is being done, nothing inside that person is heating up at all.
可以想成一個畫面：這副血自己不會變熱。身上沒有火，熱要從外面借。清晨的蛇要等太陽曬過才動得了；冷掉的引擎要跑一段才順…更多收起
可以想成一個畫面：這副血自己不會變熱。身上沒有火，熱要從外面借。清晨的蛇要等太陽曬過才動得了；冷掉的引擎要跑一段才順。英文再把這個畫面借給人：做著很傷人的事，血卻一點都沒有熱起來，心裡就是那麼靜。這個字的每個用法，都是在問一句話——熱從哪裡來，還是根本沒有熱。From the word’s history
Wiktionary records the word as cold blood plus -ed, that is, marked by cold blood. The literal picture and the modern uses sit very close together.
來源記載這個字是 cold blood（冷的血）加上 -ed，也就是「帶著冷血的」。字面畫面跟今天的用法幾乎貼在一起，所以這個字很好記。
Wiktionary (kaikki extract, entry cold-blooded): From cold blood + -ed.
Two branches
引申：血的溫度＝性子的溫度
英文常把「血熱不熱」拿來說一個人或一隻動物多容易激動。血冷，就是動起來也不會上火。
adjective · B2
Used about a person or an action: showing no feeling and no pity for the harm done to other people, and doing it calmly and on purpose rather than in a moment of anger.
冷血無情的