The case in three lines. Tara cracks her phone screen and the repair is £90. Her only savings are the £1,147.50 deposit pot, and her banking app offers to lend her £90 now for £108 next month, which is 20% for a single month. When she lists the year's surprises, she finds £100 of them in five months out of nine, and she has to write a new line on her plan.

One question to ask. "What should the new line say?" Then: "Is Priyanka right that a pot is only a label?" Let them argue both sides.

One thing to do at the table. Each person thinks back over the last few months and lists the costs that came as a surprise: a lost thing, a broken thing, a trip, a present. Add them up and divide by the number of months. That is roughly what a small rainy-day pot would need each month.

The grown-up layer. The case is about an emergency fund: surprises are not rare, so it makes sense to plan for them rather than treat each one as bad luck. It gives the friend her strongest point, that money is the same money wherever you keep it; and the repair man his, that the price is fair. The habits it models are giving every pound a job, pricing what borrowing really costs, and looking at the whole year instead of one bad month.

The case leaves the line blank on purpose. Ask for it before you offer yours.