Scriptapedia/Water Glass Demonstration

Water Glass Demonstration

This script is used to introduce stocks and flows to a group for the first time. The purpose of the script is to provide an accessible metaphor for stocks and flows that introduces the counter-intuitive nature of stocks and flows and the formal definition of a dynamic system.


Best practice

Primary nature of group task



Preparation time: 5 minutes

Time required during session: 5 minutes

Follow-up time: 0 minutes


  1. Glass
  2. Pitcher of water


Stock and flow diagram on wall (e.g., a start of a concept model or seed structure)


  • Familiarity with the counter-intuitive nature of stock-flow relationships where net flow is declining and accumulation continues to increase
  • Familiarity with the formal definition of a dynamic system where   versus a static system where  
  • Understanding that stocks and flows represent intangible quantities like frustration



  1. After drawing the first stock and flow, explain to the group that stocks and flows are counter-intuitive.
    • "Stocks and flows are counter-intuitive."
    • Say the following while pouring the water into the glass at an initially moderate rate: "Imagine that this glass is my stock of frustration, and I’m having a disagreement with [name of family member, service provider, etc.] where what they are doing is adding to my stock of frustration."
  2. Then start to gradually slow down the rate of pouring until you have just a trickle and say:
    • "Now, I say to [name], 'What you’re doing is really frustrating me,' and they listen and change their behavior."
    • "What’s happening to my stock of frustration?"
  3. Now point out that this is counter-intuitive because the net inflow is decreasing but the stock continues to rise.
    • "This is the first point that is counter-intuitive about stocks and flows and why we need this distinction."
    • "[Name] actually listened to me, but my stock of frustration continued to rise, which brings us to the second important point about stocks and flows."
    • Give examples of how we often get this wrong and point out some relevant examples such as how politicians often mistake decreasing the deficit spending as being the same as reducing accumulated debt (debt is still rising if there is deficit spending), how capping emissions of CO2 at rates where the net flow is positive will still lead to increasing levels of CO2 concentrations in the atmosphere, or how slowing the rate that risk of chronic diseases increase will still lead to a greater prevalence of chronic disease.
    • Cite the work of John Sterman on the counter-intuitive nature of even simple stock and flow dynamics.
  4. Add a few more drops to the glass while saying the following:
    • "Being frustrated with [name], I go and talk to [name of second person] who says one little thing. My reaction is to what they just did, but with all the frustration I’ve built up. And they don’t really understand why I’m so upset. I look completely irrational at this point."
    • "Now I need to drain my stock of frustration a bit."
  5. Take a drink of water.
  6. Now ask the group to identify some other flows with the glass of water.
    • "Now what are some other inflows or outflows we might consider?" Wait for a response.
    • "An outflow is spilling (my stock of frustration is overflowing)."
  7. Draw the corresponding stock and flow diagram of the glass on the wall or a flipchart. Note at this point the metaphor has shifted slightly to be just the glass of water, not the stock of frustration. The stock and flow diagram is a metaphor for the glass of water and activities of filling, spilling, and drinking from the glass, whereas during the demonstration, the glass of water was a metaphor for the psychological stock of frustration. Note, in a variation of this, one writes the variable names of the psychological stock of frustration. That works fine, but it’s adding a layer of abstraction to the example.
  8. Close the exercise by saying:
    • "So we’ve just introduced two important concepts about why we want a stock and flow distinction. First, stocks accumulate when the net flow is positive, so decreasing the net flow still adds to the stock. And second, in dynamic systems—in fact the formal mathematical definition of a dynamic system—the response or output is a function of the state of the system, not just the input to a system."

Evaluation Criteria

  • Participants quickly identify other inflows and outflows such as evaporation
  • Participants seem engaged and maybe even laugh a little when draining the stock of frustration
  • Participants refer back to the glass metaphor when subsequently discussing stocks and flows


Although it may have independently invented elsewhere, this exercise in its present form was first developed and used by Peter Hovmand.


Originally based on the bathtub metaphor as introduced by George Richardson in the "Concept Model" script, this script was developed as a way to bring in a more immediate metaphor into a classroom and introduce the idea of stock and flow dynamics around frustration as part of a group model building workshop with Ritenour High School students in 2010.




Sterman, J. D. (2000). Business dynamics: Systems thinking and modeling for a complex world: Irwin McGraw-Hill.


A variation of this script was developed by Nishesh Chaliese where, after introducing the basic stock and flow diagram, one marks a level of desired water in an empty pitcher that is the same size as a pitcher that contains water, and then sets up a “telephone ring” where one person will decide to stop pouring by saying “stop” and then the next person in the ring says “stop” and this goes around the ring until the last person tells the demonstrator to stop. During this, the demonstrator closes their eyes or wears a blindfold for dramatic effect. This can then be used to further illustrate and introduce the system dynamics diagramming conventions for goal-gap structures, information delays, and the problems that information delays can cause. This variation takes about 15-20 minutes.