In the 1980s, Paul O'Neill took over as CEO of Alcoa, one of America's largest aluminium companies. Investors expected him to announce cost-cutting. Instead, he announced he was going to focus on one thing: worker safety. Nobody would be injured on an Alcoa factory floor. Within a year, Alcoa's profits hit record highs. Within a decade, annual net income was five times what it had been when O'Neill arrived. How? Because changing safety required changing everything else. Communication had to improve so workers could report hazards. Processes had to be redesigned. Culture had to shift. One change created pressure that restructured the whole system. This is what Duhigg calls a keystone habit: a habit that, when changed, produces a cascade of other changes across multiple areas of life. Exercise is one of the most common examples. People who start exercising regularly often report that without trying, they begin eating better, sleeping better, and managing stress differently. One change unlocked the others. The question is not which habits you should change. It is which one change would make the most other changes easier.