stock and flow

Boxes for the things that hold a level, pipes with valves for the rates that fill and drain them. The split is the whole point: you never move a stock directly, only the rates on either side of it. Pipe width can carry the rate, so a slow leak becomes visible before anyone adds it up. A stock keeps its level when every rate goes to zero, which is what makes it different from a flow.

Boxes for the things that hold a level, pipes with valves for the rates that fill and drain them. The split is the whole point: you never move a stock directly, only the rates on either side of it. Pipe width can carry the rate, so a slow leak becomes visible before anyone adds it up. A stock keeps its level when every rate goes to zero, which is what makes it different from a flow.

Example

Best when the complaint is about a level that will not move, and you want to see which rate is responsible.