Bubble Popper I — Scratch L3 Lesson 17

Learnt28 min · clone, clone script, delete, count
Basic24 min · a stream of bubbles
Challenge 1–327 min · forget the delete, count them, a burst
Extrabonus · the 300 ceiling

WHAT IT SHOULD LOOK LIKE

One sprite can only be in one place. A clone is a copy of it that runs its own script — so one bubble sprite becomes a reef full of them.

one sprite

one sprite, twenty clones

panels run left to right

Review — the Firework Show arc 5 min

A My Block holds a stack under a name, and inputs let the call vary it.
Run without screen refresh finishes a definition before the Stage redraws.
Refactoring rearranges code without changing what it does.

Quick-fire

Your firework block made sixty sparks, and every one of them was the same colour even though you wanted different ones. Why could you not fix that?

Reveal the answer

Because the sparks read a shared variable, so the last one set the colour for all of them. This arc is about giving every clone its own — and this lesson is the groundwork for it.

Today’s Topic 3 min

create clone of [myself v] — a copy that runs by itself
when I start as a clone — the hat only clones run
delete this clone — and why forgetting it breaks the game
Counting how many are alive, and the ceiling Scratch puts on it

Learning outcome

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

Make many copies of one sprite and give them all a job.
Have each copy tidy itself away when it is finished.
Keep a count of how many are alive right now.

Learnt — one sprite, many bubbles 28 min

1 · The problem with one sprite

A sprite is in one place at a time. Send a bubble up from the seabed and you have one bubble; send another and the first has to come back down first.

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch three bubbles go up, one after another. Never two at once.

One sprite, three trips. A reef needs twenty at once, and twenty sprites is not the answer.

2 · A clone is a copy that runs itself

You met clones in Level 2 and used them for firework sparks. Here is what is actually happening:

create clone of [myself v] makes a copy of the sprite, exactly where it is, with everything it has.
The copy immediately runs any when I start as a clone script — its own run of it, separate from every other clone.
The original carries on with what it was doing.


when flag clicked
hide
repeat (12)
  go to x: (pick random (-180) to (180)) y: (-170)
  create clone of [myself v]
  wait (0.35) seconds
end

The spawner. It hides, and only ever makes copies.


when I start as a clone
show
repeat until <(y position) > (165)>
  change y by (5)
end
delete this clone

One bubble’s whole life, run twelve times over.

WHAT IT SHOULD LOOK LIKE

Every clone runs its own copy of `when I start as a clone`. Twelve clones is twelve scripts running side by side, each minding its own bubble.

twelve, all rising

Runs here in the page. Nothing to install.

How to use this: Click the green flag. Twelve bubbles rise at once, and the counter shows how many are alive.

One sprite made all of these. Each is running its own copy of the same six blocks.

3 · Every clone must tidy itself away

delete this clone removes the clone that runs it — not all of them, just that one. It belongs at the end of the clone script, when the bubble has finished:


when I start as a clone
change [bubbles v] by (1)
show
repeat until <(y position) > (165)>
  change y by (5)
end
change [bubbles v] by (-1)
delete this clone

Up by one when it is born, down by one when it dies. The counter then shows how many are alive, not how many there have ever been.

WHAT IT SHOULD LOOK LIKE

A clone that reaches the surface deletes itself. The counter goes up when one is born and down when one dies, so the number settles instead of climbing.

one leaving at the top

4 · What clones share, and what they do not

A clone starts with a copy of everything the sprite had: its costume, its size, its position, its effects. From that moment it goes its own way — moving one clone does not move the others.

But variables are still shared, and that is the big surprise waiting in the next lesson. For today, every bubble does the same thing at the same speed, and that is fine.

Basic — a stream of bubbles 24 min

Start a new project. A reef backdrop and one round sprite.

Step 1 — the sprite

Add the Ball sprite. It has five colours as costumes.
Set its size to about 40.

Step 2 — the spawner

Under the green flag: hide, then go to x: (0) y: (-175).
Add repeat (26).
Inside: go to a random x at the seabed, then create clone of [myself v], then a short wait.

Step 3 — the bubble’s life

Add when I start as a clone.
show, then repeat until <(y position) > (165)> holding change y by (5).
Finish with delete this clone.

Step 4 — count them

Make a variable bubbles, set it to 0 under the flag, and show it.
Bump it up at the top of the clone script and down just before the delete.
Run it and watch the number rise and settle rather than climbing for ever.

Step 5 — make them look different

In the spawner, add switch costume to (pick random (1) to (5)) before the clone block.
In the clone script, add set size to (pick random (30) to (55)) %.


when flag clicked
hide
set [bubbles v] to (0)
set [made v] to (0)
repeat (26)
  go to x: (pick random (-180) to (180)) y: (-170)
  switch costume to (pick random (1) to (5))
  create clone of [myself v]
  change [made v] by (1)
  wait (0.28) seconds
end

The spawner. Everything about how a bubble behaves is in the clone script, not here.

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch a steady stream. The bubbles box settles; the made box only climbs.

What you are aiming at. Twenty-six made, and all of them gone again by the end.

There is one bubble sitting at the bottom that never moves.

That is the original sprite. It needs hide under the green flag — clones copy the hidden state and then show themselves.

Nothing appears at all.

The clone script is missing its show, so every clone is hidden like the original.

The bubbles box climbs and never comes down.

The change [bubbles v] by (-1) is missing, or it is below the delete — where it never runs.

They all appear at the same spot.

The go to x: () y: () is inside the clone script instead of the spawner, so every clone moves to the same new place.

CHALLENGE 1 · L3-17-M1🪙 30 Adcoins8 min

Forget the delete

Break it on purpose, so you recognise it when it happens by accident.

Take the delete out of your clone script.
Run it and watch the counter.
Write down what happens to the project after a while.

WHAT IT SHOULD LOOK LIKE

Challenge 1 — take the delete out and nothing ever leaves. The count only climbs.

sixty, and stuck

Runs here in the page. Nothing to install.

How to use this: Click the green flag. Sixty clones are made and not one of them ever leaves.

The counter only goes up. On a real game this is what “it gets slower and then stops” looks like.

Done when: you can describe, without looking, what a game with no delete this clone does after a minute of play.

Teacher note

This is the single commonest clone bug and it never announces itself — the game just degrades. Getting pupils to cause it deliberately once is worth more than a warning.

CHALLENGE 2 · L3-17-M2🪙 50 Adcoins9 min

How busy did it get?

Track the most bubbles that were ever on screen at the same time.

A bubbles counter that goes up on birth and down on death.
A most at once that only ever goes up.
Change the spawn wait and watch the peak change.

WHAT IT SHOULD LOOK LIKE

Challenge 2 — counting what is alive, and remembering the busiest moment.

the peak

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch both boxes. One settles, the other keeps its record.

Eleven alive at the busiest moment. Speed the spawner up and that number climbs.

Done when: halving the spawn wait roughly doubles the peak, and you can say why.

Teacher reveal — the “biggest so far” shape again


change [bubbles v] by (1)
if <(bubbles) > (most at once)> then
  set [most at once v] to (bubbles)
end

The same three blocks as the strongest card in Lesson 3-9 and the worst till day in 3-4. Point that out — a pupil who recognises a shape they have used before is learning the thing that actually transfers.

CHALLENGE 3 · L3-17-M3🪙 70 Adcoins10 min

A burst of ten

Instead of a trickle, send up ten at once, three times over.

Ten clones made with no wait between them.
A pause between the bursts, not inside them.
They spread across the seabed rather than stacking up.

WHAT IT SHOULD LOOK LIKE

Challenge 3 — ten at once instead of a trickle. The loop is the same; the wait is what went.

a burst of ten

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch three waves of ten come up together.

The same loop as the stream. The wait moved from inside the burst to between the bursts.

Done when: the ten arrive together rather than in a line, and the gaps between the waves are clear.

Teacher note

A loop inside a loop, and the only difference from the stream is where the wait sits. Worth asking them to predict what happens if the wait stays inside — most will say “nothing much” and be surprised.

EXTRA MISSION · L3-17-MX🪙 100 Adcoinsbonus

Find the ceiling

Scratch will not make unlimited clones. Find out where it stops — and notice how it tells you.

Ask for 350 clones as fast as you can.
Count how many you asked for and how many you got.
Say what error message Scratch gave you.

WHAT IT SHOULD LOOK LIKE

Extra — Scratch allows 300 clones and then quietly refuses. Ask for 350 and you still get 300, with no error at all.

asked for 350

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch the two numbers. One reaches 350; the other stops.

Asked for 350, got 300, and nothing at all was said about it.

Teacher note

The follow-up matters more than the limit: with deletes in place a game can make thousands of clones over a session, because only the ones alive at oncecount towards 300. That is the practical rule pupils should leave with.

Summary 5 min

create clone of [myself v] copies the sprite; the copy runs its own when I start as a clone.
Hide the original. It is the spawner, not a bubble.
delete this clone removes just that one, and every clone needs it.
Count up on birth and down on death, and the number means “alive now”.
Scratch stops at 300 clones and says nothing. Only the living ones count.

Clone: A copy of a sprite that runs its own scripts.
Spawner: The hidden original whose job is to make clones.
Clone script: Everything under when I start as a clone.
Population: How many clones are alive at this moment.

Hand in

Save your reef. Next lesson every bubble gets its own speed and its own size — and you find out why that was impossible until now.

Learnt28 min · clone, clone script, delete, count
Basic24 min · a stream of bubbles
Challenge 1–327 min · forget the delete, count them, a burst
Extrabonus · the 300 ceiling

WHAT IT SHOULD LOOK LIKE

One sprite can only be in one place. A clone is a copy of it that runs its own script — so one bubble sprite becomes a reef full of them.

one sprite

one sprite, twenty clones

panels run left to right

Review — the Firework Show arc 5 min

A My Block holds a stack under a name, and inputs let the call vary it.
Run without screen refresh finishes a definition before the Stage redraws.
Refactoring rearranges code without changing what it does.

Quick-fire

Your firework block made sixty sparks, and every one of them was the same colour even though you wanted different ones. Why could you not fix that?

Reveal the answer

Because the sparks read a shared variable, so the last one set the colour for all of them. This arc is about giving every clone its own — and this lesson is the groundwork for it.

Today’s Topic 3 min

create clone of [myself v] — a copy that runs by itself
when I start as a clone — the hat only clones run
delete this clone — and why forgetting it breaks the game
Counting how many are alive, and the ceiling Scratch puts on it

Learning outcome

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

Make many copies of one sprite and give them all a job.
Have each copy tidy itself away when it is finished.
Keep a count of how many are alive right now.

Learnt — one sprite, many bubbles 28 min

1 · The problem with one sprite

A sprite is in one place at a time. Send a bubble up from the seabed and you have one bubble; send another and the first has to come back down first.

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch three bubbles go up, one after another. Never two at once.

One sprite, three trips. A reef needs twenty at once, and twenty sprites is not the answer.

2 · A clone is a copy that runs itself

You met clones in Level 2 and used them for firework sparks. Here is what is actually happening:

create clone of [myself v] makes a copy of the sprite, exactly where it is, with everything it has.
The copy immediately runs any when I start as a clone script — its own run of it, separate from every other clone.
The original carries on with what it was doing.


when flag clicked
hide
repeat (12)
  go to x: (pick random (-180) to (180)) y: (-170)
  create clone of [myself v]
  wait (0.35) seconds
end

The spawner. It hides, and only ever makes copies.


when I start as a clone
show
repeat until <(y position) > (165)>
  change y by (5)
end
delete this clone

One bubble’s whole life, run twelve times over.

WHAT IT SHOULD LOOK LIKE

Every clone runs its own copy of `when I start as a clone`. Twelve clones is twelve scripts running side by side, each minding its own bubble.

twelve, all rising

Runs here in the page. Nothing to install.

How to use this: Click the green flag. Twelve bubbles rise at once, and the counter shows how many are alive.

One sprite made all of these. Each is running its own copy of the same six blocks.

3 · Every clone must tidy itself away

delete this clone removes the clone that runs it — not all of them, just that one. It belongs at the end of the clone script, when the bubble has finished:


when I start as a clone
change [bubbles v] by (1)
show
repeat until <(y position) > (165)>
  change y by (5)
end
change [bubbles v] by (-1)
delete this clone

Up by one when it is born, down by one when it dies. The counter then shows how many are alive, not how many there have ever been.

WHAT IT SHOULD LOOK LIKE

A clone that reaches the surface deletes itself. The counter goes up when one is born and down when one dies, so the number settles instead of climbing.

one leaving at the top

4 · What clones share, and what they do not

A clone starts with a copy of everything the sprite had: its costume, its size, its position, its effects. From that moment it goes its own way — moving one clone does not move the others.

But variables are still shared, and that is the big surprise waiting in the next lesson. For today, every bubble does the same thing at the same speed, and that is fine.

Basic — a stream of bubbles 24 min

Start a new project. A reef backdrop and one round sprite.

Step 1 — the sprite

Add the Ball sprite. It has five colours as costumes.
Set its size to about 40.

Step 2 — the spawner

Under the green flag: hide, then go to x: (0) y: (-175).
Add repeat (26).
Inside: go to a random x at the seabed, then create clone of [myself v], then a short wait.

Step 3 — the bubble’s life

Add when I start as a clone.
show, then repeat until <(y position) > (165)> holding change y by (5).
Finish with delete this clone.

Step 4 — count them

Make a variable bubbles, set it to 0 under the flag, and show it.
Bump it up at the top of the clone script and down just before the delete.
Run it and watch the number rise and settle rather than climbing for ever.

Step 5 — make them look different

In the spawner, add switch costume to (pick random (1) to (5)) before the clone block.
In the clone script, add set size to (pick random (30) to (55)) %.


when flag clicked
hide
set [bubbles v] to (0)
set [made v] to (0)
repeat (26)
  go to x: (pick random (-180) to (180)) y: (-170)
  switch costume to (pick random (1) to (5))
  create clone of [myself v]
  change [made v] by (1)
  wait (0.28) seconds
end

The spawner. Everything about how a bubble behaves is in the clone script, not here.

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch a steady stream. The bubbles box settles; the made box only climbs.

What you are aiming at. Twenty-six made, and all of them gone again by the end.

There is one bubble sitting at the bottom that never moves.

That is the original sprite. It needs hide under the green flag — clones copy the hidden state and then show themselves.

Nothing appears at all.

The clone script is missing its show, so every clone is hidden like the original.

The bubbles box climbs and never comes down.

The change [bubbles v] by (-1) is missing, or it is below the delete — where it never runs.

They all appear at the same spot.

The go to x: () y: () is inside the clone script instead of the spawner, so every clone moves to the same new place.

CHALLENGE 1 · L3-17-M1🪙 30 Adcoins8 min

Forget the delete

Break it on purpose, so you recognise it when it happens by accident.

Take the delete out of your clone script.
Run it and watch the counter.
Write down what happens to the project after a while.

WHAT IT SHOULD LOOK LIKE

Challenge 1 — take the delete out and nothing ever leaves. The count only climbs.

sixty, and stuck

Runs here in the page. Nothing to install.

How to use this: Click the green flag. Sixty clones are made and not one of them ever leaves.

The counter only goes up. On a real game this is what “it gets slower and then stops” looks like.

Done when: you can describe, without looking, what a game with no delete this clone does after a minute of play.

Teacher note

This is the single commonest clone bug and it never announces itself — the game just degrades. Getting pupils to cause it deliberately once is worth more than a warning.

CHALLENGE 2 · L3-17-M2🪙 50 Adcoins9 min

How busy did it get?

Track the most bubbles that were ever on screen at the same time.

A bubbles counter that goes up on birth and down on death.
A most at once that only ever goes up.
Change the spawn wait and watch the peak change.

WHAT IT SHOULD LOOK LIKE

Challenge 2 — counting what is alive, and remembering the busiest moment.

the peak

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch both boxes. One settles, the other keeps its record.

Eleven alive at the busiest moment. Speed the spawner up and that number climbs.

Done when: halving the spawn wait roughly doubles the peak, and you can say why.

Teacher reveal — the “biggest so far” shape again


change [bubbles v] by (1)
if <(bubbles) > (most at once)> then
  set [most at once v] to (bubbles)
end

CHALLENGE 3 · L3-17-M3🪙 70 Adcoins10 min

A burst of ten

Instead of a trickle, send up ten at once, three times over.

Ten clones made with no wait between them.
A pause between the bursts, not inside them.
They spread across the seabed rather than stacking up.

WHAT IT SHOULD LOOK LIKE

Challenge 3 — ten at once instead of a trickle. The loop is the same; the wait is what went.

a burst of ten

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch three waves of ten come up together.

The same loop as the stream. The wait moved from inside the burst to between the bursts.

Done when: the ten arrive together rather than in a line, and the gaps between the waves are clear.

Teacher note

EXTRA MISSION · L3-17-MX🪙 100 Adcoinsbonus

Find the ceiling

Scratch will not make unlimited clones. Find out where it stops — and notice how it tells you.

Ask for 350 clones as fast as you can.
Count how many you asked for and how many you got.
Say what error message Scratch gave you.

WHAT IT SHOULD LOOK LIKE

Extra — Scratch allows 300 clones and then quietly refuses. Ask for 350 and you still get 300, with no error at all.

asked for 350

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch the two numbers. One reaches 350; the other stops.

Asked for 350, got 300, and nothing at all was said about it.

Teacher note

Summary 5 min

create clone of [myself v] copies the sprite; the copy runs its own when I start as a clone.
Hide the original. It is the spawner, not a bubble.
delete this clone removes just that one, and every clone needs it.
Count up on birth and down on death, and the number means “alive now”.
Scratch stops at 300 clones and says nothing. Only the living ones count.

Clone: A copy of a sprite that runs its own scripts.
Spawner: The hidden original whose job is to make clones.
Clone script: Everything under when I start as a clone.
Population: How many clones are alive at this moment.

Hand in

Save your reef. Next lesson every bubble gets its own speed and its own size — and you find out why that was impossible until now.