Water $\left(\mathrm{H}_{2} \mathrm{O}\right)$ and carbon dioxide $\left(\mathrm{CO}_{2}\right)$ are produced in a chemical reaction when methane $\left(\mathrm{CH}_{4}\right)$ and oxygen $\left(\mathrm{O}_{2}\right)$ are combined and heated. Which are the reactants, and which are the products?