The dentist told me one of my molars has gone bad.
Tips
memory
The character 臼 is itself a pictograph of a mortar — the four small strokes inside represent grain. Its molar-shaped hollow gives the medical term 臼齿 ('mortar-tooth' = molar).
usage
As a free word, 臼 is mostly literary/historical. Common compounds: 石臼 (stone mortar), 臼齿 (molar), 杵臼 (pestle and mortar), 跳出窠臼 ('jump out of the rut' — break a stale pattern).
