Nir Eyal opens by presenting his 'Hook Model' — a four-stage cycle (trigger, action, variable reward, investment) that explains how many successful digital products build habitual, repeated user engagement.

Why this predictable cycle matters for understanding product design:

• Understanding this cycle explains why certain products become genuinely habit-forming while others, despite similar functionality, fail to build sustained engagement
• This framework provides product designers a structured methodology for building genuinely engaging products, while also providing users insight into how their own engagement is being deliberately shaped
• Eyal's framework, drawn from behavioral psychology research, provides both a design methodology and, implicitly, a critical lens for understanding digital product engagement

This opening establishes the book's foundational Hook Model framework, providing structure for its subsequent detailed examination of each cycle stage.