凝固
níng gù · HSK 7
to congeal
🧊 凝 níng (congeal) + 固 gù (solid) = liquid sets into a solid
凝 carries the sense of coagulating or condensing; 固 means firm or solid. Together they mean to freeze or solidify. The figurative use, rapt attention, is a gaze that has set firm and stopped moving.