A man is hit with a baseball bat.

In his last year of school the author is hit in the face with a baseball bat. His nose breaks, his skull fractures, and he has a seizure on the field. He spends the following season recovering and does not make the team.

At college he starts going to bed early, keeping his room tidy and lifting weights on a set schedule. By his final year he is captain and is named in a national list of student athletes.

He then sets out a method. A habit has four parts: a cue, a craving, a response and a reward. To build one, make the cue obvious, make it attractive, make it easy and make it satisfying. To break one, invert each of the four.

Examples follow. A British cycling coach named Brailsford changes a large number of small things and the team wins the Tour de France. A stockbroker named Trent Dyrsmid moves paper clips from one jar to another, one clip for each sales call, and moves 120 a day. Japanese train drivers point at signals and say out loud what they see. A man puts an exercise bike in front of the television and cycles only while watching.

Further terms are introduced. Habit stacking, where a new habit is attached to one that already exists. Temptation bundling, where a wanted activity is paired with a required one. The two-minute rule, where the habit is cut down until it takes two minutes. The plateau of latent potential, where an ice cube sits in a room warming by one degree at a time and then melts.

The closing chapters cover genetics, boredom, and the difference between a goal and a system. There is a summary of every chapter at the back and a list of further reading.