The case in three lines. Tara paid £350 for ten driving lessons, has had four, and is now offered one lesson a fortnight with her test in June. She means to wait, so as not to waste £210, until the terms show unused lessons are refunded less a £30 fee. Switching to a weekly instructor costs £48 more than the refund, against twelve weeks of waiting, and the reply box is open on her phone.

One question to ask. "What should Tara write to the driving school?" Then: "Is her dad right that changing instructor wastes a lesson?" Let them argue both sides.

One thing to do at the table. Think of something someone at the table kept doing only because they had already paid for it or started it, such as a box set, a gym month or a book. Split it the way Marcus does: what was already spent, and what is still to come. Ask whether the first part should have mattered.

The grown-up layer. The case separates a sunk cost, money already spent that no choice can bring back, from the costs that are still ahead, and shows how an exit that feels expensive can be cheap once it is read. It gives Tara's dad his strongest point, that switching has a real cost of its own, and gives the instructor his: he is honest, cheap and good, just busy. The habits it models are reading the exit terms, splitting money into gone and still to come, and comparing choices by what each one costs from now on, in time as well as money.

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