Synonym playground
6 short pages, each written with deliberately non-overlapping vocabulary -- one says "couch" and never "sofa", another says "sofa" and never "couch" -- so a query only crosses between them because synonym expansion ran, not because the words happen to be related some other way. "sofa" ↔ "couch" is a symmetric equivalence class; "laptop" → "notebook" is directional (try searching "notebook" with expansion on: it won't pull in the laptop page, but searching "laptop" pulls in the notebook page). See synonyms for how the mechanism works.