How it’s done
Save the next big box that comes into your house — the one the new fridge came in, the one from the TV — and hand it to your child with no instructions. That's where it all begins.
The only rule is not to direct: don't tell him "let's make a car." Ask "what is it?" and follow his answer. Your job is production assistant, not director.
Tools nearby: crayons, scissors (supervised), tape, scraps. The box gets painted, gets cut, gets windows opened in it — and when it falls apart, nobody cries: you recycle it and wait for the next one.
What it builds — the why
Imagination with no script — the most valuable kind, because it doesn't come with a correct use built in at the factory. An open-ended object forces the child to invent the whole game: what it is, what it does, who he is inside it. That leap — from "this is for this" to "this can be anything" — is pure creative thinking.
How it changes with age
0–2 Babies
3–5 Early childhood
6–9 Childhood
Variations
Siblings version: a box for each one, connected by tunnels — a cardboard neighborhood. Moving version: the moving boxes are weeks of free play while you unpack the suitcases.
What to watch for in your child
Notice whether your child needs the box to "be something" right away or enjoys the box as a box for a good while before deciding. Neither is wrong — but the second kind needs you to resist the temptation to suggest what to do. The silence before the idea is part of the work.