The Tea Urn (Process Modelling)
Process modelling is a way of tracking how something changes over time — the same basic idea applies whether it's water in a tank, money in a bank account, energy in a battery, or cups of tea in an urn. Every version comes down to two things: a stock (however much of something you currently have) and the flows adding to it or draining it. It's useful for understanding how a process behaves over time, for planning ahead when you can influence some levers (like when to refill) but not others (like demand), and for seeing how a delay between a decision and its effect can throw off even a reasonable plan. You're running the tea counter at a busy café. A 20-cup urn has to stay stocked through opening hours, but customer demand follows a rough rush-hour pattern that's never quite the same two days running — busier some mornings, quieter some lunches, always with a bit of a surprise.
It can take time for the kitchen to brew and deliver more tea — drag the lead-time slider below to see what that delay does to your plan, and set how many cups to add at each two-hour checkpoint. Then click "Run the day" and see how it held up. Run dry too early and customers go without; refill too generously and tea goes to waste before it's poured; order too close to a rush (or too close to closing) and it won't arrive in time to help at all. "New day" rolls a fresh, slightly different rush pattern to plan against — "Reset plan" clears your schedule back to zero.
What this teaches: Schedule refills for a tea urn against unpredictable demand and watch the stock level change over time. It teaches stock-and-flow process modelling, feedback and delay, and why delays can wreck a reasonable-looking plan.