We want to be able to take derivatives of functions ``one piece at a time.'' The {{CMD_89}}sum rule{{CMD_90}} allows us to do exactly this. The sum rule says that we can add the rates of change of two functions to obtain the rate of change of the sum of both functions. For example, viewing the derivative as the velocity of an object, the sum rule states that to find the velocity of a person walking on a moving bus, we add the velocity of the bus and the velocity of the walking person.