The bunches of three words around the outside of the puzzle all have a third word they naturally pair with to form a common word or phrase, considering only how they sound. For example, RECORDER, SCOTCH and WORM all match with TAPE to form TAPE RECORDER, SCOTCH TAPE and TAPEWORM, while BEST, GOOD and POLAR all match with BUY/BI/BYE to form BEST BUY, GOODBYE, and BIPOLAR. The puzzle is structured like a fractal: you can do the same thing to work further inwards. For example, TAPE, AQUA and CON match with DUCT to form DUCT TAPE, AQUEDUCT and CONDUCT.

The answer in the heart of the puzzle is AIR. In retrospect, ERE, ERR or E’ER could also work, so the puzzle’s a little ambiguous, though the AIR spelling is used in AIR DUCT, AIRBALL and AIRPLANE.