膝盖
xī gài · HSK 7
knee
🦵 膝 (knee) + 盖 (cover/cap) = the kneecap, the bone that covers the knee
盖 means lid or cover, the same character in 盖子 (a lid). So 膝盖 literally pairs the knee joint with the cap that covers it, matching the English word kneecap. Sound-alike for gài: rhymes with guy.
盖 — grass 艹 + 盍 (cover/lid)
Originally written 葢, this character combines the grass radical 艹 with 盍 ('to cover'), which itself shows a lid over a vessel. It first referred to a thatched grass cover or lid, then broadened to mean 'to cover' generally, and later extended to surnames and the conjunction 'thereupon'.