唇

Bottom 口 is the indexing mouth radical - a small square depicting the lips parted to speak or eat. It locks 唇 onto the body part it names: the lips frame the mouth, so the mouth radical sits beneath the phonetic top. Same family of mouth-anatomy and mouth-action chars: 吻 to kiss, 喉 throat, 舌 tongue, 吃 eat.

Top 辰 supplies the sound - chén drifting to chún with regular nasal-coda shift. 辰 originally pictured a clam or shellfish opening its shell, which gives a faint visual fit with parted lips. Same phonetic spawns 振 to shake, 晨 dawn, 娠 pregnant - though only 唇 and 蜃 mussel keep the chún-rhyme.