← Thinking in Loops · Lesson 1 of 4
Stocks and flows
A bathtub is the whole idea in one place. The water in the tub is a stock: an amount you could measure at any moment. The tap and the drain are flows: rates that change the stock over time.
A stock can only change through its flows. If the tap runs faster than the drain, the water rises; if slower, it falls. If they are equal, the level holds steady even though water is moving the whole time.
Once you look for them, stocks are everywhere: money in an account, fish in a lake, people waiting in a queue, trust between two neighbours. Each one fills and empties at rates, and each one takes time to change.
The bathtub
Water in the tub: starts at 20, ends at 98, highest 98.
Try this: Make the tap and the drain equal. The water stops changing, even though water is still moving.
What this model assumes
- The water in the tub changes only by what comes in and what goes out. (well established)
- The tap and the drain run at a steady rate. (a guess)
A reservoir is half full. Rain adds water faster than the town draws it out, but the town draws more each year. What happens to the level this year?