To understand how elementary steps make up a mechanism, that the rate law for an elementary step (unlike the rate law for an overall reaction) can be determined without experiment, and how to write the rate law for an elementary step (just by analyzing its balanced equation). Very often, a reaction does not tell us the whole story. For instance, the reaction does not involve a collision between an NO₂ molecule and a CO molecule. Based on experimental data at moderate tempertures, this reaction is thought to occur in the following two steps: