核
かく · kaku · JLPT N3 · noun
kernel
🍑🌰 核 = 木 tree + 亥 sound: a fruit's hard stone, so the core of anything
核 first meant the hard pit of a tree's fruit. From there it came to mean any core: 原子核 (げんしかく) is the atomic nucleus. 核 alone can also mean nuclear, as in 核兵器 (nuclear weapons). Hook: kaku has a hard k at each end, like a hard core.
核 — wood 木 + 亥 (phonetic)
A semantic-phonetic compound: 木 ('wood/tree') gives the meaning, while 亥 supplies the sound. It originally referred to a hard-shelled fruit pit or kernel inside wood/fruit, then extended figuratively to 'core,' 'nucleus,' and in modern usage 'nuclear' (as in atomic nucleus).