The Green Flag & Stop Sign — Scratch L1

Where this arc is heading: one green-flag click sends the cat gliding across the Stage to meet a friend sprite. The cat says hello; the stop sign halts everything cleanly. Lessons 4–6 build this scene piece by piece.

Learning Goals 3 min

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

Point to the green flag and the red stop sign above the Stage.
Explain that when ⚑ clicked waits for the flag and starts the script.
Click the stop sign mid-script and watch a long stack of moves halt immediately.

Warm-Up 7 min

Last lesson (Arc 3/6), you could find any block in the Blocks Panel. Today (Arc 4/6) you'll put those blocks to work — meeting the two buttons that run your scene: the green flag and the red stop sign.

Quick-fire puzzle

Wei Jie built this stack and clicked the green flag. Predict — how does the cat behave?


when flag clicked
move (20) steps
move (20) steps
move (20) steps
move (20) steps
move (20) steps

Five move blocks under one hat. One flag click.

Reveal the answer

The cat moved 100 steps in one big-looking jump. Scratch ran the five blocks top to bottom, so fast that the cat looked like it teleported. Each move added 20 to the cat's x — 20, 40, 60, 80, 100. The flag started the run; the cat finished before you could blink.

New Concept — start and stop 15 min

Think of a school race. The teacher waves a green flag — everyone runs. The teacher blows a whistle — everyone stops. Scratch has the same two signals: the green flag starts every script, and the red octagon stops every script. They sit side-by-side, just above the Stage.

The two buttons

Button	Where	What it does
Green flag ⚑	top-right of the Stage area	Starts every script that begins with when ⚑ clicked.
Red stop sign 🛑	right next to the green flag	Stops every running script in the project. Sprites freeze mid-action.

Hat blocks listen for events

A hat block is shaped like a hat with a rounded top. It can't go anywhere except the very top of a stack. Its only job is to wait for an event — and then let the script underneath run.

when ⚑ clicked waits for the green flag to be clicked.
Other hat blocks (you'll meet them in lessons 16 and 17) wait for a key press, or a sprite click, or a message.
Without a hat, a stack just sits there. Click the flag — nothing happens. The script has no "go" signal.

How the green flag and the hat fit together

Here's the chain of events when you click the green flag:

You click the green flag ⚑ above the Stage.
Scratch shouts "flag clicked!" to every sprite.
Every when ⚑ clicked hat block in the project wakes up.
The blocks underneath each hat run top-to-bottom.

And the stop sign?

The red octagon does the opposite. When you click it, Scratch interrupts every running script. Sprites freeze where they are. Sounds cut off. The project is paused until you click the flag again.

Why it matters

Some scripts will get long — a dance routine, a story, a chase scene. If something goes wrong (the cat runs off the edge, the music never ends), you need an instant kill switch. That's the stop sign. Knowing both buttons makes Scratch feel safe to experiment with.

Worked Example — build a long stack, then stop it 15 min

Open Scratch in a new tab. The cat is on the Stage. Today we build a deliberately long script so the stop sign has something to interrupt.

Stage plan. The cat (✏️ badge — the only sprite you script today) sits centre-left. Click ▶ to run; click ■ to stop. Both controls sit above the Stage in Scratch.

Step 1 — Drag the hat block

From the yellow Events category, drag when ⚑ clicked into the Script Area. This is the only hat we know so far.

Step 2 — Snap six move blocks underneath

From the blue Motion category, drag move (10) steps and snap it under the hat. Now grab another. And another. Snap six in total, all with 10 steps.

Step 3 — Add a wait between each one

This is the trick. From the orange Control category, drag wait (1) seconds. Snap one between each pair of move blocks. You'll alternate: move, wait, move, wait, move, wait, move.

Your stack should look like the picture below. The wait blocks turn a teleport into a stroll.


when flag clicked
move (10) steps
wait (1) seconds
move (10) steps
wait (1) seconds
move (10) steps
wait (1) seconds
move (10) steps

Eight blocks total — the L1 cap. The cat now moves 10 steps every second, four times.

Step 4 — Click the green flag

Watch the Stage. The cat hops 10 pixels right. Pauses. Hops again. Pauses. Four hops, three waits, about three seconds in total.

Step 5 — Now click the green flag and IMMEDIATELY click the stop sign

Run the script again. Halfway through (after the first hop or two), click the red octagon next to the flag. The cat freezes right there. The remaining move blocks never run.

What changed: compared to lesson 1's instant two-block script, this stack takes time. That makes the stop sign actually useful — there's something for it to interrupt.

The full assembled stack (your reference)


when flag clicked
move (10) steps
wait (1) seconds
move (10) steps
wait (1) seconds
move (10) steps
wait (1) seconds
move (10) steps

Your finished Worked Example. Click the flag to start. Click the stop sign to halt.

Try It Yourself — three small builds 12 min

🟢 Easy

Goal: Make the cat hop three times with a wait between each hop. Click the flag to run it. Then click the flag again and stop it after the first hop.


when flag clicked
move (30) steps
wait (1) seconds
move (30) steps
wait (1) seconds
move (30) steps

Think: Without the wait blocks, all three moves would happen in one instant. The waits give the stop sign a window to actually catch the script mid-run.

🟡 Medium

Goal: Build a script that hops three times, but the first wait is short (0.5 seconds) and the second wait is long (2 seconds). Click the flag — feel the rhythm.


when flag clicked
move (30) steps
wait (0.5) seconds
move (30) steps
wait (2) seconds
move (30) steps

Think: The number inside wait ( ) seconds can be a decimal. Try 0.1 for a quick flicker, or 3 for a long pause.

🔴 Stretch

Goal: Make a "stop me if you can" script. The cat moves 5 steps, waits 1 second, moves 5 steps, waits 1 second — a slow shuffle. Can you click the stop sign exactly between the second and third move? The cat should end with x somewhere around 10.


when flag clicked
move (5) steps
wait (1) seconds
move (5) steps
wait (1) seconds
move (5) steps
wait (1) seconds
move (5) steps

Think: Check the cat's x in the Sprite Properties panel after you stop it. Did you catch it exactly at 10? Or did it sneak past? The stop sign is fast — but your finger has to be faster.

Mini-Challenge — the freeze game 5 min

"Aisyah's musical statues"

Build a script that makes the cat hop slowly across the Stage. A friend (or your teacher) calls "FREEZE!" at a random moment. You must hit the red stop sign before the next hop. The cat's final x decides the winner.

It works if:

The script uses only blocks from today's lesson and SCR-L01-01.
The cat moves at least three times when nobody yells "FREEZE!".
You can demonstrate stopping the cat mid-walk with the red stop sign.

Reveal one valid solution

Use waits long enough to give your friend a chance to shout. One second feels right. Four hops gives roughly four FREEZE windows.


when flag clicked
move (20) steps
wait (1) seconds
move (20) steps
wait (1) seconds
move (20) steps
wait (1) seconds
move (20) steps

Score the round by the cat's final x. Closer to 0 means the stopper acted fast; closer to 80 means the cat almost finished.

Recap 2 min

Today you met the two buttons above the Stage: the green flag starts every script that begins with when ⚑ clicked, and the red stop sign halts everything instantly. You also learned the wait (1) seconds block — a tiny piece of Control that turns instant teleports into visible motion.

Green flag (⚑): The button above the Stage that starts every script beginning with a flag hat block.
Stop sign (🛑): The red octagon next to the flag. Clicking it halts every running script at once.
Hat block: A block with a rounded top. Sits at the very top of a script. Listens for an event before running.
wait (block): A Control block that pauses a script for a number of seconds before continuing.

Homework 1 min

The Catch-the-Cat Challenge. Build a 7-block script that walks the cat across the Stage with waits between each step. Practise stopping it at a precise spot.

Open Scratch. Set the cat's x to -200 in the Sprite Properties panel.
Build this exact stack: when ⚑ clicked · move (40) steps · wait (1) seconds · move (40) steps · wait (1) seconds · move (40) steps · wait (1) seconds.
Click the flag. Try to hit the red stop sign when the cat reaches as close to x = 0 as you can.
Note the cat's final x value from the Sprite Properties panel.

Bring back next class:

A screenshot of your Script Area showing the 7-block stack.
Three attempts written down: "Attempt 1: x = ____. Attempt 2: x = ____. Attempt 3: x = ____. Closest to 0 was ____."

Heads up for next class: SCR-L01-05 teaches you how to pick a new sprite from the library — say goodbye to "only the cat" forever.