In other words, the concentration of these guys and this guy remains the same even though the reactions are occurring. And that's because they're occurring at the same rates. Now, suppose we have the following combustion reaction in which methane combusts in the presence of oxygen to produce carbon dioxide and water. Suppose this reaction is allowed to go to completion. Now, what happens if two moles of methane of this guy react with six moles of water? So what happens?