I'm confused by the application of this law to food. My reason: excrement.<p>You put some mass that contains chemical energy into your body. Then that mass comes out of your body, minus SOME of the chemical energy. How much of it is extracted? What determines this? I have no idea. That said, I think it is funny to treat it as if the only way to get rid of energy is expenditure. How about:<p>Energy intake - Energy Ependiture - Energy still in food when it comes out the other end = Change in energy stores