Nature and life
These characters pair living things and natural forms: 巣 is a nest, 彗 a broom or comet, 舜 a morning glory, 孔 a cavity, 髄 the living core or marrow, and 巴 a swirling comma design. Link them through images of shelter, growth, openings, and inner life rather than one single category.
巣, 彗, and 孔 each have 11, 11, and 4 strokes; use the counts to notice how differently simple and dense forms are built.