Stock and Flow

Stock is a quantity that is measured by its current level.

Flow is a quantity that is measured by the rate at which it changes.

I find it useful in life to think about whether things are stock items or flow items, and whether they should be stock items or flow items. For example, food is something I want to be flow, not stock, because effectively using food is to bring it into the house (in-flow), cooking it, and eating it (out-flow). On the other hand, shop tools are mostly stock. I can measure whether I have enough tools at a given moment, and I don't really want to be constantly adding and removing tools.

If you try to use a flow item like a stock item, it causes problems-- e.g. food rotting in the fridge. If you try to use a stock item like a flow item, it causes problems-- e.g. over-consuming by constantly buying and purging shop tools or home decor.