Several parts combine into one character.

Pictograph: an open mouth showing teeth in upper and lower rows. Oracle bone forms display the teeth grid clearly. Three forms with substantial divergence: 繁體 齒 / 新字体 歯 / 简体 齿. Both Japan and Mainland China simplified, but went separate directions — another case where reading across CJK scripts requires knowing all three forms.

Note 齒 (teeth) is distinguished from 牙 (fang / molar — earlier batch) — 齒 covers the front teeth visible when smiling; 牙 is the rear molars / animal fangs. In everyday speech the distinction blurs but it's real in formal vocabulary.

Mandarin: chǐ, dipping 3rd tone (simplified 齿). 牙齿 (yáchǐ, "teeth" — combines 牙 + 齿 for the everyday word), 齿轮 (chǐlún, gear / cog — "tooth wheel"), 切齿 (qièchǐ, gnash one's teeth). Modern Mandarin tends to use 牙 alone for casual "teeth" and 牙齿 for the formal compound; 齿 alone appears in literary or technical contexts.

Japanese: on-reading シ (shi) for compounds — 歯科 (shika, dentistry), 歯科医 (shikai, dentist), 永久歯 (eikyūshi, permanent teeth), 乳歯 (nyūshi, baby teeth), 義歯 (gishi, dentures). Kun-reading は (ha) is the everyday word — 歯 (ha, tooth), 歯医者 (haisha, dentist), 虫歯 (mushiba, cavity / tooth decay — "bug-tooth"), 歯磨き (hamigaki, toothbrush / brushing teeth — "tooth-polish"). 歯ブラシ (haburashi, toothbrush) uses katakana for "brush".

虫歯 (mushiba, "bug-tooth" = cavity) is one of the most charmingly literal Japanese compounds — every Japanese child encounters this word during dental hygiene lessons.

Memory aid: a mouth showing rows of teeth — the picture is the meaning, simplified across three scripts.