The case in three lines. Tara has saved £180 for noise-cancelling headphones, and the shop offers three years' cover for £39. Read aloud, the leaflet covers only years two and three, not dropping or losing them. It also says about one customer in twenty claims, for an average repair of £60. That makes the risk worth about £3, and Tara has to answer at the till.

One question to ask. "What should Tara say?" Then: "Is paying not to worry ever worth it?" Let them argue both sides.

One thing to do at the table. Think of something in the house you could buy cover for: a phone, a bike, the boiler. Guess the chance it breaks and what a repair would cost. Multiply them and compare the answer with the price of the cover.

The grown-up layer. The case is about expected value: a risk's worth is its chance times its size. It also separates the losses you could survive from the ones you could not. It gives the shop assistant his strongest point, that peace of mind is a real thing people pay for, and gives Tara's mum hers, that her washing-machine cover paid out. The habits it models are reading the small print aloud, finding what you already have from the maker and the law, and checking whether the cover touches the thing you are actually afraid of.

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