反应
fǎnyìng · HSK 3
response
↩️ 反 = reverse + 应 = respond → a response that comes back
反 means reverse/opposite and 应 means respond/answer. A 反应 is literally a 'responding-back' — the return signal a system gives when something acts on it, hence reaction or response.
反 — hand 又 under cliff 厂
Originally a pictograph of a hand (又) turning something over against a cliff or slope (厂), depicting the action of flipping or reversing. From this came the core meaning 'to turn over,' which extended to 'opposite,' 'contrary,' and 'rebel against.' The character is purely semantic, with no phonetic component.
应 — 广 (shelter) + 心 (heart) + 隹 (bird)
Originally written 應, this character combined a phonetic component with 心 (heart) to suggest 'responding from the heart' or 'answering a call'. The simplified form 应 keeps the top portion. Over time it broadened from 'respond/answer' to also mean 'should' or 'ought to'.