The case in three lines. Tara sees a regular saver paying 6% and works out that it will earn her £91.80 on this year's £1,530. Because she pays in £127.50 a month, it would really earn about £50 over a full year, and her deposit is due in August. Closing the account early pays 1.5%, about £6, against about £17 in her easy-access account, and the clerk hands her the form.

One question to ask. "What should Tara write in the box?" Then: "Is a locked account worth it for the discipline alone?" Let them argue both sides.

One thing to do at the table. Pick something someone at the table is saving for. Write down the date the money is needed and how much goes in each month. Count the payment-months up to that date the way Cyrus does, and see what a rate really pays on it.

The grown-up layer. The case separates a quoted yearly rate from what a year of monthly payments earns, and shows how a fixed term interacts with when the money is needed. It gives Hana her strongest point, that being unable to touch the money is itself worth something, and gives Tara's dad his, that a few pounds may not be worth the complication. The habits it models are reading the line under the big number, adding up payments one at a time, and choosing the account by what the money is for and when.

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