作者提出了一套理论来解释成功的 app，尤其是成功的聊天类的app为何让用户上瘾： The Hook is composed of a trigger, action, variable reward, and investment.
- Trigger: A trigger is what cues a habit.
- Action: Notifications prompt users to act, in this case tapping the app.
- Variable Reward: This is when users get what they came for and yet are left wanting more.
- Investment: The final phase of the Hook prompts the user to put something into the service to increase the likelihood of using the service in the future. For example, when users add friends, set preferences, or create content they want to save, they are storing value in the platform.