Before you start the clock

One honest correction before day one. The "21 days to build a habit" idea everyone quotes doesn't come from habit research at all: it traces back to a 1960s plastic surgeon who noticed amputees took about three weeks to adjust to a missing limb. Actual research out of University College London, tracking real people building real habits, found the average is 66 days, sometimes far longer.

The same UCL research found missing a single day along the way doesn't meaningfully derail the process. So if you miss one, you already know what to do: Chapter 17 covered it. It's a lapse, not a verdict.