The cookie is one possible explanation

The source in context.

The appendix changes the reading mode from following an explanation to conducting a small investigation. Duhigg uses his own afternoon cookie routine to work through four tasks: identify the routine, experiment with rewards, isolate the cue, and make a plan. Possible rewards are treated as competing explanations, not guessed once and assumed correct. His notes record place, time, emotion, other people and the preceding action, making the search for a cue visible. The final plan preserves the social break while changing the trip for a cookie. The account includes missed attempts and adjustments. The book closes its practical contract by showing how uncertainty can be investigated rather than hidden.

What this example supports

The appendix converts a model into a sequence: identify the routine, test possible rewards, isolate the cue, and make a plan. The crucial move is treating the first explanation of your behavior as something to investigate.

Its limit

The appendix calls this a practical starting point, not a comprehensive formula. This small self-observation case is not a controlled demonstration or a substitute for appropriate help with serious behaviors.