Much of systems thinking can be introduced with a bathtub. The water in the tub is a stock: an amount that exists at a moment. The tap and the drain are flows: rates that add to or take from it over time.
The stock changes only through its flows. That sounds obvious, and yet it explains a good deal of surprise. A level can keep rising after the inflow has been turned down, because the inflow is still larger than the outflow. Turning the tap is not the same as the level falling.
Levels move slowly
Stocks give systems their memory and their inertia. They smooth out sudden changes in the flows and take time to fill or drain. This is why some problems persist long after their cause has eased, and why some remedies take longer to show than anyone hoped.
Turning the tap is not the same as the level falling.
Asking two questions
Faced with an accumulation, it helps to ask what is flowing in and what is flowing out, and which of the two can actually be changed. The answer is often less dramatic than the level itself suggests.