---
title: 'How this book works'
part: 'Part 0 — Start Here'
partNumber: 0
order: 2
subtitle: 'Running cells, the scrubber, and checkpoints'
---

Every page of this book mixes prose and runnable code. You do not need anything installed — the compiler runs right here in your browser.

## The Run cell

A **Run cell** is an editable code editor with a live preview below it. Edit the code, then press **Run ▶** (or **Cmd/Ctrl+Enter**) to see the result immediately. The **Reset** button restores the original code.

Try it:

<Run>
  {`// Draw a square
repeat 4 [
  fd 20
  rt 90
]`}
</Run>

Change `20` to `40` and run again. Change `4` to `6` and `90` to `60`. Every number you change is an experiment.

## The Scrub cell

A **Scrub cell** adds a playback scrubber below the preview. Drag the slider left to rewind stitch by stitch — or press **▶** to watch the design build up in real time. The highlighted line in the editor shows which line of code produced the current stitch.

<Scrub>
  {`repeat 8 [
  fd 20
  rt 45
]`}
</Scrub>

Scrub cells are especially useful when the book teaches _order_ concepts — buffered satin columns, trimming, fill regions.

## Read-only cells

Some cells are **read-only** — they show a finished example without inviting edits. Click **Edit ✎** to fork the cell into a full editable version and experiment freely.

## Checkpoints

Each chapter ends with a **Checkpoint**. Complete the task described, then click **Mark complete ✓**. Your progress is saved in this browser and shown as checkmarks in the sidebar.

<Checkpoint chapterId="ch-0-2">
  Run both cells on this page. Drag the scrubber in the Scrub cell all the way to the left (0
  stitches), then play it back. When you understand how the scrubber works, mark this checkpoint
  complete.
</Checkpoint>