The air you're breathing right now is the free raw material for one of the deepest-moated businesses on Earth. Gas companies capture air, chill it into a liquid at –196°C, then "distill" it into oxygen, nitrogen, and argon — the gases hidden inside every ton of steel, every chip, and every patient's breath. The remarkable part: the whole world is run by just 3–5 players, through pipes and plants that, once built, almost no one can ever steal the customer away from.