Adding the Pen Extension — Scratch L4

Where this arc is heading: a Pen Art Studio where one cat-paintbrush draws polygons, a Deepavali kolam, and a spirograph — all from short, clever loops. Every lesson adds one drawing skill until the Studio runs on its own.

Learning Goals 3 min

By the end of this lesson you will be able to:

Explain what an extension is in Scratch 3 — extra block palettes that aren't loaded by default.
Use the bottom-left Add Extension (blue +) button to add the Pen extension and confirm the new green palette appears.
Name the seven Pen blocks and what each one does at a glance — without running any drawings yet (we draw next lesson).

Warm-Up — count the palettes 7 min

Open a fresh Scratch project right now. Look at the column of coloured dots down the left side of the Code area — each dot is a palette. Count them.

Which colours do you see, top to bottom?

Reveal the answer

There should be exactly nine: Motion (blue), Looks (purple), Sound (pink), Events (yellow), Control (orange), Sensing (light blue), Operators (green), Variables (orange-red), and My Blocks (pink-red). Nine palettes. That's everything Scratch loads by default — every block you've ever used in Levels 1, 2, and 3 lives in one of those nine.

But notice the bottom-left corner. There's a blue square with a + icon and the label Add Extension. That button is the hint: there's more. There are blocks Scratch ships with that aren't in the default nine — they live behind that + button, and you have to add them per-project.

Today we add our first one: the Pen extension. After today, your project will have ten palettes — the ninth being a new green one full of drawing blocks. Next lesson we'll actually draw with them.

New Concept — extensions and the Pen palette 15 min

Scratch 3 is built around the idea that the default nine palettes shouldn't be all there is. Some users want to control robots. Some want to translate to Malay. Some want to draw. Putting all those blocks in the default palette would make the editor cluttered and intimidating for first-day users. So Scratch splits the optional blocks into extensions — bundles you turn on per-project.

What an extension is

An extension is a named group of extra blocks that lives behind the Add Extension button. When you add an extension to your project, a new palette appears in the Code area below the My Blocks palette. The blocks behave just like the built-in ones — they snap, they run, they reporter — they were just sleeping until you woke them up.

Scratch ships with about a dozen extensions out of the box. A few you'll meet eventually:

Pen — draw with sprites. Today's lesson.
Music — play notes and drum beats.
Text to Speech — make sprites speak out loud through your speakers.
Translate — translate strings between languages.
Video Sensing — react to webcam motion.
Hardware extensions — micro:bit, LEGO, makey-makey, and others, if you have the kit plugged in.

Extensions are per-project. If you add Pen to Project A, and then make a new Project B, Project B starts back at the default nine palettes. (When you open Project A again later, the Pen palette comes back automatically — extensions are saved as part of the project file.)

How to add Pen

Bottom-left of the editor, click the blue square with the + icon. A picker fills the screen showing every available extension as a tile.
Click the Pen tile (it has a picture of a pencil and a green colour-bar).
You're snapped back to the editor. Look at the palette column on the left — there's now a green dot at the bottom. That's Pen.
Click the green dot. The Pen palette opens. Seven blocks.

The seven Pen blocks — at a glance

This is the survey. We are not drawing anything yet — we're just naming the blocks and what each one is for. Next lesson you'll put your first pen down on the Stage.

erase all — wipes every pen mark off the Stage. The Stage goes blank. Often the first block in any drawing script, so old drawings don't pile up.
stamp — prints a copy of the sprite's current costume onto the Stage at the sprite's current position. The sprite itself keeps going; the stamp stays. Great for trails and patterns.
pen down — the sprite starts drawing a line wherever it moves. It's like lowering a pencil tip onto paper.
pen up — the sprite stops drawing. It still moves; it just doesn't leave a line. Like lifting the pencil.
set pen color to () — sets the line colour. The slot is a colour picker — click it to choose any colour from a wheel.
change pen color by () — shifts the colour by a number along the rainbow. Useful for rainbow spirals: small change each loop = smooth rainbow.
set pen size to () — sets the line thickness in pixels. 1 is hairline, 100 is fat marker, 0 hides the line.

(Pen also has change pen size by () — a partner to set-pen-size. Some counts say "eight blocks" because of this pair. Either way, it's a small palette and you'll learn all of them this cluster.)

The mental model

Imagine every sprite is holding an invisible pencil. By default, the pencil is up — it doesn't touch the Stage. When you run pen down, the pencil presses down. From that moment, every move () steps, glide, or change x/y draws a line behind the sprite. When you run pen up, the pencil lifts again — the sprite can move freely without leaving marks.

That's the whole drawing model in two blocks: down to start, up to stop. The other five blocks just customise what the line looks like and let you wipe the Stage clean. Next lesson we'll draw our first shape with exactly four blocks.

Common mistakes

"Pen isn't there!" — You opened a brand-new project and there's no green palette. That's expected. Extensions are per-project; you have to click the + and add Pen every time you start a fresh project.
Looking for Pen in the Looks palette. Pen is its own palette, below My Blocks. Looks is for costume changes and speech bubbles, not drawing.
Confusing stamp with pen down. Stamp prints the sprite's costume once. Pen down draws a line as the sprite moves. Different blocks, different effects. Today, just remember the names.
Forgetting erase all at the top of your script. Without it, every flag-click adds more drawings on top of yesterday's drawings. The Stage gets messy fast. Habit: every pen script starts by erasing.
Setting pen size to 0 by accident. 0 means "no line at all". You'll move the sprite, expect to see a line, and see nothing. Check the size — it should be at least 1.

Worked Example — installing Pen and exploring the palette 12 min

This lesson's example is a tour, not a drawing. We're going to add the extension, look at every block, and write the setup stack we'll use next lesson — without yet pressing the flag to draw anything. This is the very first piece of our Pen Art Studio.

Stage plan. The cat (the only sprite you script — see the ✏️ badge) becomes our paintbrush, parked dead-centre on a blank canvas. Today we only set it up; next lesson the pen goes down and the canvas fills.

Step 1 — Start a fresh project

From scratch.mit.edu, click Create. New empty project. Default cat. Count the palettes — nine, as expected.

Step 2 — Click the Add Extension button

Bottom-left of the Code area. Blue square with a white + icon. Click it. The extension picker fills the screen — tiles for Music, Pen, Video Sensing, Text to Speech, Translate, and a few hardware ones.

Step 3 — Click the Pen tile

Look for the tile with the pencil icon and the green colour-bar. Click it. You'll bounce back to the editor.

Step 4 — Confirm the new green palette

Look at the column of coloured dots on the left. Scroll all the way down. Below My Blocks there's now a green dot labelled Pen. Click it. Seven (or eight) blocks appear — the Pen palette.

Step 5 — Hover over each block

Don't drag yet. Just hover. Read each block's words. Compare them to the list in the concept section above. You should be able to point at each block and say what it does without looking it up.

Step 6 — Build a "setup" stack (no drawing yet)

Click the cat sprite. Drag a when flag clicked. Underneath, drag erase all, then set pen color to () (click the colour slot, pick blue), then set pen size to (4). The stack looks like:


when flag clicked
erase all
set pen color to [#1e90ff]
set pen size to (4)

The "I'm about to draw" setup. Clears the Stage, picks a blue colour, picks a medium thickness. No pen down yet, so still no drawing.

Step 7 — Click the flag (and notice nothing happens)

You'll see... nothing. No line, no change. That's correct. You haven't put the pen down and you haven't moved the sprite. The setup happens silently. Setup blocks usually do — they prepare the Stage without showing off.

Step 8 — Save the project as your Pen sandbox

File → Save now (or wait for the auto-save). Name it Pen-Sandbox. We'll come back to this project for the next several lessons of Cluster B. Because the extension is saved with the project, when you re-open it tomorrow, the green Pen palette will already be there.

The full setup stack


when flag clicked
erase all
set pen color to [#1e90ff]
set pen size to (4)
go to x: (0) y: (0)

A slightly bigger setup stack — clears the Stage, sets a colour and size, and parks the sprite at centre. Five blocks. Next lesson, we add pen down + a movement block and watch the line appear for the first time.

What you just did: you turned the Pen extension on, met every block in the new palette, and built the standard opening five-block setup that almost every Pen project starts with. Everything else in Cluster B is just adding to this foundation.

Try It Yourself — three Pen-palette drills 15 min

🟢 Easy

Goal: In your Pen-Sandbox project, open the Pen palette and drag each of the seven Pen blocks onto the Code area (just floating, not snapped to anything). Arrange them in a row. Click each one once to make it flash — every block lights up when you click it, even if it doesn't visibly do anything.


erase all
stamp
pen down
pen up
set pen color to [#000000]
change pen color by (10)
set pen size to (4)

The seven Pen blocks lined up. Not snapped together — that wouldn't make sense yet. Just floating, side by side, so you can read them and click each one.

Think: Clicking a single block runs just that block. erase all wipes the Stage, which you'll see immediately. The others mostly do invisible things — set up state, lift the pen. That's why this lesson doesn't draw anything yet: most Pen blocks need a movement block after them to show their effect.

🟡 Medium

Goal: Make a second new project (so you start fresh with nine palettes). Add two extensions this time: Pen and Music. Confirm two new palettes appear at the bottom of the column — green for Pen, pink for Music. You don't have to use the Music blocks; this is about proving extensions stack.

Think: A project can have as many extensions on as you want. Each one adds its own palette. The default-nine grows to ten, eleven, twelve. Most real projects only need one or two extras — adding everything just clutters the palette.

🔴 Stretch

Goal: Use last lesson's See-Inside habit. Find a shared Scratch project that uses the Pen extension. (Search "spiral" or "fractal" or "kaleidoscope" — these usually do.) See Inside. Confirm the green Pen palette is in the palette column. Read the project's main script and find every Pen block it uses. Write the block names in your notebook.


when flag clicked
erase all
go to x: (0) y: (0)
pen down
repeat (100)
  move (5) steps
  turn cw (15) degrees
  change pen color by (3)
end
pen up

A typical small Pen project you might find while searching — a rainbow spiral. Uses five of the seven Pen blocks: erase all, pen down, change pen color by, and pen up (with set-pen-color hidden inside the colour-changing loop). Don't build this yet — just spot it in the wild.

Think: Most Pen-extension projects share the same skeleton: erase, set up, pen down, loop a movement + turn, pen up. You're learning to recognise that skeleton today so when we build our own next lesson, the structure feels familiar instead of new.

Mini-Challenge — "Where did the green palette go?" 5 min

The disappearing extension

Faiz spent yesterday's class adding Pen to a project and building a setup stack. Today he opens a brand-new Scratch project from the homepage to start something different. He clicks on the palette column expecting to see Pen, but it's not there — only the nine default palettes. He's sure he did the steps yesterday. What happened?


when flag clicked
erase all
set pen color to [#ff8800]

Faiz's script from yesterday — he had this working with Pen blocks in green. But today, the Pen palette is gone and he can't drag those blocks anymore. What's going on?

Reveal one valid solution

Faiz hasn't lost his work — he's just in a different project. Extensions are per-project, not per-account. When you add Pen to Project A, it stays in Project A forever (saved as part of the project file). But a brand-new Project B starts from the default nine palettes — Pen is not automatically added.

The fix is simple, once you know:

If Faiz wants to continue yesterday's project: go to My Stuff, find yesterday's project (probably called "Untitled-N"), open it. The Pen palette is right there because it was saved with that file.
If Faiz wants to start something new with Pen: in the new project, click the bottom-left + button and add Pen again. Two clicks. The green palette comes back. He'll have to do this for every fresh project.

The bigger lesson: extensions belong to projects, not to you. They're part of the saved file. Open the file, the extension is there. Start a new file, you start fresh. Once you know this, the disappearing palette stops feeling mysterious.

Recap 3 min

Scratch 3 loads with nine default palettes — Motion, Looks, Sound, Events, Control, Sensing, Operators, Variables, and My Blocks. The Add Extension button in the bottom-left corner (blue + icon) unlocks extra palettes that aren't loaded by default. Today you added the Pen extension and a new green palette appeared at the bottom of the column — seven Pen blocks: erase all, stamp, pen down, pen up, set pen color to (), change pen color by (), and set pen size to (). Extensions are saved per-project, so a fresh project starts back at nine palettes. Next lesson, we put the pen down and draw our first line.

Extension: A named group of extra blocks that Scratch ships with but doesn't load by default. You add an extension per-project via the bottom-left + button; a new palette appears below My Blocks.
Pen extension: The Scratch extension that lets sprites draw on the Stage as they move. Adds a green palette with seven blocks. The foundation of every line drawing, spiral, fractal, and stamping pattern you'll build in Cluster B.
Palette: A coloured group of related blocks in the Scratch editor — Motion is blue, Looks is purple, Pen is green. Each palette has its own coloured dot in the column on the left of the Code area.
Pen down / Pen up: The two blocks that switch the sprite's invisible pencil on and off. With pen down, every movement draws a line behind the sprite. With pen up, the sprite moves silently with no line. The most-used pair of Pen blocks.
Stamp: The Pen block that prints a copy of the sprite's current costume onto the Stage at the current position. Different from pen down (which draws a thin line) — stamp leaves a whole sprite-shaped picture.
Erase all: The Pen block that wipes every pen mark and every stamp off the Stage. Almost every Pen script starts with this block, so old drawings from previous flag-clicks don't pile up.

Homework 2 min

The Pen-Palette Tour. Open your Pen-Sandbox project (or start a new one and add the Pen extension again). For each of the seven Pen blocks:

Drag the block onto the Code area as a single floating block (no hat, no stack).
Click the block once to make it flash.
In your notebook, write one sentence: "This block does X, but I won't see it until I also do Y." (For example: "set pen color to blue does change the colour, but I won't see it until I also pen down and move.")

Seven blocks × one sentence = seven lines in your notebook. The point isn't to draw anything yet — the point is to understand which blocks have visible effects on their own (only erase all and stamp) and which need movement to show off (all the others).

Bring back next class:

Your seven-line notebook entry.
Your Pen-Sandbox project saved with at least the four-block setup stack from Step 6.
Your answer to: "Which Pen block do you most want to use first when we start drawing next lesson, and what would you try to draw with it?"

Heads up for next class: SCR-L04-07 is Pen Down, Pen Up — we finally press the pen down, move the sprite, and watch our first line appear. Bring your Pen-Sandbox project ready to extend.