Yes, thermodynamics. However, if you eat five hundred calories per day, you will a) feel miserable, b) be constantly thinking of food, and c) perhaps die, even if there is fat hanging around in the body to be used up. This demonstrates that we're not idealised food-burning machines, but food-burning machines which adjust their desire to take in fuel depending on how the machine is operating. It is very, very difficult to stand a low-calorie diet for more than a few weeks, if the diet is administered in the wrong way with the result that you crave more food all the time. By contrast, some people (like me, and several others on this thread) find it day-to-day easy to eat below maintenance if carbs are cut out. (Although personally I have never been able stand no-carb for more than about a month, so no-carb doesn't really work for me if I want to lose weight.)