A very simple question, but incredibly hard to answer without thinking about it. In fact, I don't think I can give a correct answer without looking up. Even then, I would probably be wrong. Luckily, this great article will help me in the future. It gives enough evidence and answers simple questions like "why fire is hot regardless of the fuel" and "why in spite of its high bond enthalpy, O2 is abundantly present in our atmosphere.?"