I took a controls systems course with Dr Guy Longobardo. He was a great lecturer who could make tough concepts intuitively easy to grasp. One day he was explaining the major parameters affecting controlling and got to "demand side capacity": the ability of the destination of a process to react quickly to changes from the controller. The example he used was taking a shower, pointing out that if you turn on more hot water the water coming from the showerhead quickly becomes hotter. He said that was because the showerhead has a "small demand side capacity" and the small amount of water inside it can be quickly replaced by the new hotter water. That was such a perfect analogy that even this morning, a half century later, as I was taking a shower and adjusting the temperature, the phrase "small demand side capacity" jumped into my mind as it has been doing every shower since that lecture.