The Hook Model: trigger, action, reward, investment

Eyal's entire framework rests on a four-step cycle that habit-forming products repeat until usage becomes automatic. A trigger (external, like a notification, or eventually internal, like boredom) prompts an action (the simplest behavior done in anticipation of a reward). That action produces a variable reward, and finally the user makes a small investment — data, content, effort, or reputation — that increases the odds they'll return and loads the next trigger.

Eyal stresses that the cycle must be completed repeatedly before a habit forms; a single satisfying experience doesn't create a habit, but dozens of quick, low-friction loops do. He points to how quickly checking a phone becomes reflexive not because any single check is compelling, but because the loop has run thousands of times.

The model's power is that after enough repetitions, the trigger becomes internal — a feeling of boredom or loneliness alone is enough to prompt the action, with no external notification needed.

Takeaway: map any habit you're building (in a product or in yourself) onto trigger, action, reward, investment to see which step is weak.