This is about TDD more than design.
Create the business logic for a small app, that for now (and maybe forever) has no UI (no Command-Line Interface, no web application).
A Trick-or-Treater wears a costume, and carries a bag.
The Trick-or-Treater knows what they're dressed up as.
You can put pieces of candy in the bag. You can find a particular type of candy in the bag, if it is there.
The trick or treater can grab one or more pieces of candy from the bag. You can count the number of pieces of candy in the bag.
The Trick-or-Treater eats candy, and based on the amount of sugar in the candy, their blood sugar rises. At first, Trick-or-Treaters are relaxed, but as they consume more candy, they become excited, hyper-active, and finally they fall into a sugar-induced coma.