One of the most persistent challenges in obesity research is explaining why weight loss decelerates over time, even when dietary adherence is maintained. This phenomenon, commonly called adaptive thermogenesis, is more accurately termed metabolic adaptation, as it reflects broader reductions in energy expenditure than heat production alone. Here, I propose that this phenomenon is not a distinct biological compensation, but an inevitable physical consequence of mass loss. Building on the mass balance model (MBM), I show that the rate of mass loss follows Torricelli's Law – the same principle that describes how a water tank empties. The square-root relationship arises analytically from the body surface area–mass connection and predicts weight loss trajectories during prolonged fasting without an explicit "adaptation" term. Reframing metabolic adaptation through Torricelli's Law offers a more parsimonious, first-principles explanation for one of the most robust observations in human metabolism.