還元
かんげん · kangen · JLPT N1 · noun
reduction
還 = return + 元 = origin: return to the origin, reduce back to it
In chemistry, 還元 means reduction, where a substance gives back electrons. Resolving a problem is also a return to its original state. Both senses come from 還 (return) and 元 (origin).
元 — 二 (two) + 兒 variant 儿 (legs/person)
元 originally depicted a person with the head emphasized on top, meaning 'head' or 'origin'. From this came the extended senses of 'first', 'beginning', and 'primary'. It was later borrowed as the name of the Yuan dynasty and the modern Chinese currency unit.