The case in three lines. Tara practises a first week's shop for the flat with £30, and her mum's rule is that the big pack is always cheaper. The shelf tags tell a mixed story: the big cereal box costs more per hundred grams than the regular one, the big bag of pasta costs less, and the big bag of salad is cheaper per gram until you count how much of it a person living alone will actually eat. Tara has to write a rule at the top of her list.

One question to ask. "What should Tara's rule say?" Then: "Was her mum's rule wrong?" Let them argue both sides.

One thing to do at the table. Next time you are in a shop, pick one thing that comes in two sizes. Find the small price on the shelf tag for each, the price per kilogram or per hundred grams, and see which one is really cheaper.

The grown-up layer. The case is about unit price: comparing the price per hundred grams rather than the price of the pack, and then thinking about how much will actually be used before it goes off. It gives the mother her strongest point, that "buy big" mostly works for a family of four. It gives the friend hers: some multi-buys really are cheaper. The habits it models are reading the small print on the shelf tag, comparing like with like, and pricing what you will use rather than what you buy.

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