Vertical Slices
Product page through API through database to a visible product, instead of a week of database then a week of backend. A slice is independently testable; a skeleton proves the pieces connect.
Not the database in week one, the backend in week two and the frontend in week three — but one product, from page to API to database and back, visible on day one. A slice is one narrow feature through every layer, and it is the unit in which a system actually becomes real.
Building layer by layer feels safe because each layer is a real thing that gets finished. The trap is that a finished layer proves nothing about the product, and every question about the product is deferred to the last week, when they all arrive together.
Browser → API → Database → Browser, doing almost nothing, working end to end, deployed. A walking skeleton is the thinnest possible slice built to prove the pieces connect — and it deliberately proves nothing else.
Create Product → Store Product → Display Product: three steps that together are the smallest thing a store can do that is worth watching. Find it, build it, watch it run, then grow it — because every grown step is tested against something that already works.
Create Order through Frontend → API → Business Logic → Database → Response can be tested on its own, before any other slice exists. That is what makes a slice a unit of progress rather than a unit of work — and the test must say what it does not prove as clearly as what it does.