I don't think an analogy that focuses on "capacity" alone is a great one. You want an analogy with both stocks and flows. It's worth checking out Donella Meadow

I don't think an analogy that focuses on "capacity" alone is a great one. You want an analogy with both stocks and flows. It's worth checking out Donella Meadows' book "Thinking in Systems: A Primer". here's a rough disk space analogy with stocks and flows: Suppose you have a disk with free space X. What rate do you consume free disk space? What rate do you produce more free disk space? if flow out (of disk space) is larger than flow in (of disk space) then after some finite time you will always run out of disk, no matter how much free space X you started with. More capacity (a larger initial value of X) just buys you more time until the emergency. Which is useful, but you still haven't solved the actual problem in terms of the delta between the two flow rates. Now if you have your flow rates under control, and you produce at least as much free disk space as you consume, what if the flow rates aren't always steady, but have variability? Then having some spare capacity can help to provide slack or buffer to deal with temporary periods where disk is consumed much quicker than usual. But there are diminishing marginal returns to having lots of excess capacity. > Annual income twenty pounds, annual expenditure nineteen six, result happiness. Annual income twenty pounds, annual expenditure twenty pound ought and six, result misery. --Charles Dickens