Breathing: an act that is for most of us perpetual, essential, and completely thoughtless. Our constant exchange of interior and exterior, oxygen for carbon dioxide (or, if we happen to be plants, the opposite) goes largely unnoticed, even if the average adult human lungs hold six liters and are emptied and filled twelve to twenty times every minute. We notice respiration most when things go wrong -- when airways are blocked, when maintaining breath requires medical intervention, or when the atmosphere becomes toxic following disaster or slow degradation of air quality in some murky but worrisomely plausible future. Or we may choose to notice, focusing on our breathing in meditation and psychological exercise. Meanwhile there are those who, instead, deny breath, pushing their physiological limits. And all around us, gas exchange occurs on larger scales: nitrogen into the soil, carbon into the ocean, the entire planet taking its own essential breaths.