Abstract
My seventh-grade science teacher asked the class to list "the three most essential needs of human life." The "correct" answer-water, food, and sleep-illustrates how easily people take for granted the air we breathe, specifically its oxygen content. Most humans can live a few days without water, food, and sleep, and yet we can't go more than a few minutes without oxygen. I understood that much in junior high, but at the time I had no idea that Earth's oxygen-rich atmosphere represents-and facilitates-a miracle.