Willpower is like a muscle because, while people’s total amount of it is limited, they can still increase it over time. Thus, habits don’t just
save willpower—they can also help
build it. In fact, one way to increase it is through keystone habits. Namely, people can build willpower in one domain (like children in sports or music), then easily transfer it to another. This is why Starbucks trains such successful employees: the willpower they learn in training easily transfers to the challenges they face on the job.