What's the milkshake's job?

What's the milkshake's job? 

In the mornings, customers who bought the milkshake alone from the fast food chain said that they bought it to accompany them for the long commute to work. They've tried other alternatives like a bagel or a banana but none does its job as well as the milkshake. The bagel was too boring on its own so customers had to spread butter or jam that they placed between their legs while driving. It sometimes became messy. The banana did its job too quickly. Customers finished the banana really quickly and soon, they were left hungry before lunchtime. 

In the afternoons, the milkshake did another job. Parents often ordered two meals to go for themselves and their child. Children love milkshake. As compared to saying yes to a new toy or to go to a new place for the afternoon, saying yes to a milkshake was the easiest and cheapest. 

When you know what's the milkshake's job, it's easier to boost sales for the milkshake.

In relationships, we all have a job. If your job for your friend is to be a listening ear but one day you decided to go to her house, ignore what she wants to tell you and do her chores for her, you might lose a friend. Similarly, if our spouse wants us to do the chores but all we do is keep talking, a quarrel will ensue. 

How Will You Measure Your Life? by Clayton M Christensen, James Allworth and Karen Dillion contains this and other stories that show us how we can use theories in business to find and create happiness in real life.