This script is used to introduce concepts of stock and flow to those without previous knowledge of system dynamics. The purpose of the script is to provide an easy to understand metaphor for the mentioned concepts.
Primary nature of group task
Preparation time: 5 minutes Time required during session: 5 to 10 minutes Follow-up time: 0 minutes
- Two transparent glasses or cups.
- One or two packs of cookies which in total contain 20 of them; rectangle shaped cookies are preferred.
A Stock symbol (from stock and flow diagram) on a whiteboard or a flipchart.
- Familiarity with the concepts of stock and flow, and their relationships where a positive net flow accumulates a stock and a negative one drains it.
- Modeler, facilitator, or gatekeeper with training in system dynamics modeling and simulation
- Unpack cookies on a table. Tell the group: “Let’s assume that each cookie is a file/task/duty/homework that should be handled”
- Now, put a glass on the table and draw the attention of your audience to the empty glass.
- Draw a rectangle on the whiteboard/flipchart. Do not tell them the name yet.
- Go back to the table and while putting the first cookie, with a low pace, into the glass, tell: “Every day we receive these tasks and duties.”
- Continue filling the glass with cookies one by one. Tell the group: “assume we do not do anything with this tasks and postpone them”
- When the glass fills and overflows with cookies, stop. Explain that wherever something accumulates, where it is possible to make a snapshot and measure/count the accumulated quantity, we can name it a “stock”. Make sure to mention that this notion is different from the thing that people hear usually in stock markets, especially if your audience are from finance and/or business sector.
- Put the other empty glass to the right side of the first one, within a reachable distance from you.
- Go back to the whiteboard and draw the second stock (do not draw the flow between them yet).
- Tell: “but, in reality, we have to handle those tasks and pass them to our colleague/ teammate”. At the same time, take one of the cookies and move it to the second glass. Tell: “As you see, there is flow of tasks from us to our colleague/ teammate”. Continue this action till both glasses contain equal number (or height) of cookies.
- Go back to the whiteboard and draw the flow between two stocks, “we call this a flow”
You can stop the presentation here, because your audience are now familiar with the concept of stock and flow. However, it is better to continue the presentation.
- Ask two participants to join you and stand to the left and right side of you. Both glasses still have same number of cookies. Ask the one who is in front of the first glass to add cookies to the first glass one by one and ask the second participant to pick cookies from the second glass with the same pace. You, the presenter, follow the pace and pick one cookie from the first glass and put it into the second one.
- Continue till the pile of cookies finishes. Tell the group: “There is no more task! As you see, we do not receive any other task and the first stock/glass is becoming empty”
- You and the second participant continue till the first glass becomes empty.
- Ask the second participant continue removing cookies from the second glass till the glass becomes empty.
- Now you can explain that how you “drained” the stocks.
- Thank the participants and ask them to return to their seats.
- Close the exercise, “we just get familiar with the concept of stock and flow. We just learnt that flows fill or drain stocks. If the inflow is larger than the outflow, the stock accumulates/overflows. If the outflow becomes larger than the inflow, stock drains or becomes empty”.
- For the fun and also ice-breaking, serve the participants with those cookies during the first break. Don’t be stingy!
- Participants identify other inflows and outflows such as outsourcing or dismissing the duty/task.
- Participants seem engaged and maybe laugh when draining the stock of cookies.
- Participants may refer back to the cookie and glass metaphor when subsequently discussing stocks and flows.
This exercise was first developed and used by Davood Qorbani in Nijmgen, the Netherlands (2016).
This text is a variation of bathtub metaphor by George Richardson in the “Concept Model” script. Furthermore, the text is indebted to “Water Glass Demonstration” script by Peter Hovmand as a reference for choosing suitable wordings, and borrowing many sentences especially in “output” and “evaluation criteria” sections of this script.
Richardson, G. P. (2013). Concept models in group model building. System dynamics review, 29(1), 42-55. Sterman, J. D. (2000). Business dynamics: systems thinking and modeling for a complex world. Boston: McGraw-Hill Higher Education.