Every morning, millions of people glance at their wrists and receive a number that feels like truth — a precise accounting of energy spent, effort made, progress earned. Yet the calorie figures displayed by smartwatches are, at best, educated approximations, shaped by formulas designed for a statistical average that describes almost no living person. The gap between the confidence of the display and the uncertainty of the underlying science is not a flaw to be patched in the next software update; it is a fundamental boundary of what can be known from a sensor on the skin.