system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Where does the "heat" come from in our body?
gpt
From exothermic (heat-producing) chemical reactions in all of our cells.  Those chemical reactions are what allow the cells to stay alive.  They involve "burning" sugars such as glucose.  The reaction really is the exact same reaction you get from literally setting sugar on fire, but it's much more controlled.  The end result is that the sugar is combined with oxygen to produce water and carbon dioxide.  This is why the air we breath out has less oxygen and more carbon dioxide than the air we breathe in.