口 is a simple square — an open mouth viewed from the front, drawn in three strokes. Identical across 繁體 / 新字体 / 简体.

It is one of the most productive radicals in the entire script, generating two whole categories of compound. First, mouth-actions: 吃 (eat), 喝 (drink), 吐 (spit/vomit), 呼 (call/breathe out), 吸 (inhale), 唱 (sing), 叫 (shout), 呢 (a final particle), 吗 (a question particle). Second, "opening / passage" by metaphor: 名 (name — a mouth giving a label), 句 (sentence — what a mouth produces), 員 (member — a "mouth" inside a frame), 古 (ancient — ten generations of mouths), 周 (around).

The metaphor extends beyond the body: anywhere people or things flow in and out, Chinese imagines a "mouth". 入口 (entrance), 出口 (exit), 港口 (harbor), 人口 (population — "human mouths"), 窗口 (window/counter). When you see 口 in city signage at airports, train stations, and shopping malls, it is always marking an entrance or exit.

Mandarin: kǒu, dipping 3rd tone. Also a measure word for people in a household: 三口人 (sān kǒu rén, "three people in the family"), 四口之家 (sì kǒu zhī jiā, "a family of four mouths"). The mouth-as-headcount metaphor is built into the grammar.

Japanese: on-reading コウ (kō) in 人口 (jinkō, population), 入口 (iriguchi or nyūkō); ク (ku) in 口調 (kuchō, tone of voice). The kun-reading くち (kuchi) is the everyday word — 口 (kuchi, mouth), 入り口 (iriguchi, entrance — a kun-reading mix). 一口 (hitokuchi, "one mouthful").

Memory aid: a small square. Open mouth. The picture is the meaning.