空 opens 工 under the cave-roof 穴: the hollow within. Emptiness was first a physical cavity; the sky became the greatest hollow of all — 天空, heaven's cave.
空 splits by tone: kōng is empty and sky — 天空 (sky), 空气 (air — 'empty qi'), 空调 (air conditioning: 'air adjuster', summer's best friend). kòng is the empty slot in your schedule: 有空吗? ('got time?' — the Chinese 'are you free?'). Buddhism's deepest term also lives here: 空 (śūnyatā, emptiness) — 色即是空. From AC units to enlightenment, the hollow holds it all.
The hollow under the cave roof 穴 — EMPTY; your empty hours are FREE TIME.
空调 'air adjuster' (AC) and 空 'Buddhist emptiness' share a character — comfort and enlightenment, one hollow apart.