toy

Build the world; the human drives it. Never automate the step they are trying to learn. One behavior per world; cut every control and readout no prediction needs. Drive the real system when it is cheap to run, toy data when it is heavy. A dynamic the system does not show is labeled approximate on screen, never drawn as real.

The world shows what is hidden — the stack, the queue, the cursor, the intermediate value — at every step, scrubbable forward and back, each effect re-rendered beside its cause on every press. Room beside any step to pin a note.

Beside each control that steps or fires, room to say what it will do before pressing; the world sets the prediction beside what happened. The human reads the match; the world never scores it. A miss names the next thing to drive.

The human lacks	Build	A case that fixes it
How execution unfolds	Stepper: scrub through time, internals visible at each step	a regex engine on one input: cursor and backtrack stack at every step
What a migration, port, or bulk change does	Command center: the human fires each step, before and after side by side	a rename over forty files: one file per press, its before and after together
How a structure or algorithm behaves	Objects they can move, values live	a B-tree: drag a key in, watch the split
What a parameter or design space does	Sliders with instant re-render	a rate limiter: bucket size and refill rate, the traffic curve redraws on every nudge

Default medium: one page outside the repo, YYYY-MM-DD-<slug>.html, opened for them (open <path>). Then step out. Held is when their predictions land — theirs to report, not yours to claim. Say what the world leaves approximate.

The reader lacks what it is, not how it behaves → use grasp skill. The world is a playground, demo, or UI → not toy; say so and stop.