Launch Week III — Scratch L3 Lesson 47

Learnt28 min · the redraw rule, and where a crowd comes from
Basic24 min · dust that tidies up
Challenge 1–327 min · leftovers, the ceiling, and hiding
Extrabonus · run without screen refresh

WHAT IT SHOULD LOOK LIKE

Your engine makes a speck of dust every tick. Fifty-six of them in one flight — and if none of them ever leaves, the game runs out of clones to make.

dust everywhere

Review — finding a bug 5 min

Write the number you expect before you look at anything.
Monitors and one say beat reading a script.
Halve the project until one script is left; then change one block.

Quick-fire

Last week a loop counted to a hundred thousand in about a second. How many times was the Stage drawn while it did that?

Reveal the answer

About thirty. A hundred thousand turns of the loop, thirty pictures. Those are the two speeds a Scratch game runs at, and today is about the difference.

Today’s Topic 3 min

Why some loops run once a frame and others run thousands of times
delete this clone — and what happens to the ones you forget
Three hundred clones is the whole project’s share, not one sprite’s
run without screen refresh on a My Block

Learning outcome

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

Say why a loop ran more times than you expected.
Make hundreds of clones and leave none behind.
Spot a game that has quietly run out of clones.

Learnt — thirty pictures a second 28 min

1 · The rule under everything

Scratch draws the Stage about thirty times a second. A loop that changed something you can see waits for the next drawing before going round again — so it goes round thirty times a second. A loop that changed nothing visible does not wait.


repeat until <(timer) > (7)>
  change [seen v] by (1)
  change y by (0)
end

Moves something. 181 turns in six seconds.


repeat until <(timer) > (7)>
  change [unseen v] by (1)
end

Moves nothing. Tens of thousands of turns in the same six seconds — 15,395 on one run, 90,434 on another.

WHAT IT SHOULD LOOK LIKE

Two loops, six seconds each. The one that moves something took 181 turns — thirty a second, every time. The one that changes nothing you can see took tens of thousands, and a different number on every run.

the same six seconds

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch the two monitors race. Both loops stop at the same moment.

181 against tens of thousands. The 181 is the same every run; the other number never is, which is the point.

This is why last week’s countdown vanished, and it is why a cooldown counted in loop turns never lasts as long as it says. It is also good news: a game loop with movement in it keeps time by itself.

2 · Where a crowd comes from

Our engine makes one speck of dust per tick of thrust. That sounds tiny. A gentle descent is 180 ticks long and burns for about a third of them:


if <(costume [name v]) = [lander-thrust]> then
  set [puff x v] to (x position)
  set [puff y v] to ((y position) - (24))
  create clone of (Dust v)
end

The lander makes the clone, so no speck ever hears an order meant for the original — Lesson 3-37’s bug, avoided by not broadcasting.

WHAT IT SHOULD LOOK LIKE

One speck per tick of thrust. A gentle descent burns for about a third of its 178 ticks, so a single flight makes 56 of them.

one flight

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch the two monitors: specks made climbs all the way down, specks alive stays small.

56 specks in one flight, and four alive at the end — because each one deletes itself.

3 · Making them is not free either

create clone of (myself v) puts something new on the Stage, so it waits for the next drawing too. Two hundred clones in an ordinary loop take two hundred frames to arrive.

Runs here in the page. Nothing to install.

How to use this: Click the green flag. The same two hundred specks are made twice — first in an ordinary loop, then inside a ticked My Block.

One at a time: 6.6 seconds. All at once: 0 seconds. Same two hundred specks.

Basic — dust that tidies up 24 min

Give your own game something that appears in numbers — dust, sparks, bubbles, coins — and make sure every one of them leaves.

Step 1 — one clone per tick

Make the sprite and two variables, puff x and puff y.
In the flight loop, set those two and create clone of (Dust v).
Run it. The Stage should fill up quickly.

Step 2 — a life, then a goodbye

In when I start as a clone, go to the puff, then repeat (12) drifting and fading.
After the repeat, delete this clone. Not hide.
Run it again. The Stage should now stay tidy while the flight goes on.

Step 3 — fewer, and nobody notices

Make a speck only every third tick: if <((ticks) mod (3)) = (0)> then.
Watch it. It looks the same, and it is a third of the clones.

Runs here in the page. Nothing to install.

How to use this: Click the green flag and hold the up arrow to make dust. The two monitors count what was made and what is still alive.

What you are aiming at: a long burn, a trail of dust, and specks alive that never climbs.

Every speck appears in the same place.

The clone is reading puff x and puff y after the lander has moved on, or it never reads them at all. Set them immediately before create clone of (Dust v), and go to them as the clone’s first action.

The dust fades but the Stage still fills up.

You used hide or ghost 100 instead of delete this clone. Invisible is not gone — that is Challenge 1.

My specks alive monitor counts up and never down.

Put change [specks alive v] by (-1) directly above delete this clone. After the delete, nothing runs.

After a while no new dust appears at all.

You have hit three hundred. Everything else in your project has stopped being able to clone as well — see Challenge 2.

CHALLENGE 1 · L3-47-M1🪙 30 Adcoins8 min

The specks that never leave

This lander’s dust fades to nothing, which looks exactly like tidying up. Prove that it is not.

Watch both monitors for a whole flight.
Compare what was made with what is still alive at the end.
Work out how many flights this game would survive.

WHAT IT SHOULD LOOK LIKE

Challenge 1 — a speck fades to ghost 100 and stops being visible. It has not stopped being a clone: 56 made, 56 still alive.

faded, not gone

Runs here in the page. Nothing to install.

How to use this: Click the green flag and let it land. Read specks made and specks alive at the end — they are the same number.

56 made, 56 alive. Four more flights like that and the project is full.

Done when: you can say how many flights this project gets before it can make no more clones, and your own version keeps specks alive under twenty.

Teacher note

Ask for the division out loud: 300 divided by 56. Five flights, and then a game that stops working for reasons nothing on screen explains. This is the most common way a Scratch project mysteriously breaks after ten minutes of play.

CHALLENGE 2 · L3-47-M2🪙 50 Adcoins9 min

The explosion that never came

Three hundred is the whole project’s allowance, not one sprite’s. Here is what that looks like from the inside.

The dust asks for four hundred specks. Count what it gets.
The crash then asks for five pieces of wreckage.
Find the error message Scratch shows. There is not one.

WHAT IT SHOULD LOOK LIKE

Challenge 2 — four hundred specks were asked for and three hundred arrived. Then the crash asked for five pieces of wreckage and got none at all.

the ceiling

Runs here in the page. Nothing to install.

How to use this: Click the green flag. Read specks alive first, then watch the bits monitor when the lander switches to its crashed costume.

400 asked for, 300 made. Then five pieces of wreckage were asked for, and none appeared.

Done when: you can explain why a sprite that has never made a clone in its life can still fail to make one.

Teacher note

The silence is the lesson. A pupil whose explosion stops working will look at the explosion — and the explosion is perfect. Give them the habit of putting specks alive, or any clone counter, on the Stage while they build.

CHALLENGE 3 · L3-47-M3🪙 70 Adcoins10 min

Hidden is not gone

Same three hundred specks. This time every single one of them is hidden, so the Stage is empty — and the project is just as full.

Run it. Look at the Stage, then look at the monitor.
Ask for the five pieces of wreckage again.
Say, in one sentence, the difference between hide and delete this clone.

WHAT IT SHOULD LOOK LIKE

Challenge 3 — the same three hundred, every one of them hidden. The Stage looks empty and the project still cannot make one more clone.

hidden is not gone

Runs here in the page. Nothing to install.

How to use this: Click the green flag. Nothing appears — read the specks alive monitor, then the bits monitor after the crash.

An empty Stage holding three hundred clones, and no room for five more.

Done when: you have found any hide in your own project that should have been a delete, and changed it.

Teacher reveal — the one-line difference


when I start as a clone
show
repeat (12)
  change y by (-2)
  change [ghost v] effect by (6)
end
change [specks alive v] by (-1)
delete this clone

hide changes what a clone looks like. delete this clone ends it: its scripts stop, and its place in the three hundred is given back. Only one of those two makes room for the next one.

EXTRA MISSION · L3-47-MX🪙 100 Adcoinsbonus

Run without screen refresh

A My Block has a tick-box nobody reads. Turning it on tells Scratch to finish the whole block before drawing anything — so two hundred steps happen inside one frame.

Make a My Block that does two hundred steps of work.
Time it with (timer), twice: tick off, then tick on.
Use it for something real — building a crowd, or checking a whole list.

Runs here in the page. Nothing to install.

How to use this: Click the green flag and wait. The same two hundred steps run twice, and the sentence at the end gives both times.

Ordinary: 6.6 seconds. Run without screen refresh: 0 seconds. Identical work, identical answer.

Teacher note

Good place to name the trade the whole lesson is about: Scratch spends its time either drawing or thinking. Every trick here is a way of choosing which.

Summary 5 min

The Stage is drawn about thirty times a second.
A loop that changed something visible waits for that. One that did not, does not.
A project may hold 300 clones at once, shared by every sprite.
hide is not delete this clone.
A ticked My Block finishes before the next drawing — good for work, not for movement.

Frame: One drawing of the Stage. About thirty of them a second.
Redraw: Scratch putting the Stage on the screen again after something changed.
Clone limit: The 300 clones a whole project may have alive at one time.
Run without screen refresh: A My Block setting that finishes all its work inside one frame.

Hand in

Save your game with a clone counter on the Stage. Next week it gets a score, a board, a front door and a proper test.

Learnt28 min · the redraw rule, and where a crowd comes from
Basic24 min · dust that tidies up
Challenge 1–327 min · leftovers, the ceiling, and hiding
Extrabonus · run without screen refresh

WHAT IT SHOULD LOOK LIKE

Your engine makes a speck of dust every tick. Fifty-six of them in one flight — and if none of them ever leaves, the game runs out of clones to make.

dust everywhere

Review — finding a bug 5 min

Write the number you expect before you look at anything.
Monitors and one say beat reading a script.
Halve the project until one script is left; then change one block.

Quick-fire

Last week a loop counted to a hundred thousand in about a second. How many times was the Stage drawn while it did that?

Reveal the answer

About thirty. A hundred thousand turns of the loop, thirty pictures. Those are the two speeds a Scratch game runs at, and today is about the difference.

Today’s Topic 3 min

Why some loops run once a frame and others run thousands of times
delete this clone — and what happens to the ones you forget
Three hundred clones is the whole project’s share, not one sprite’s
run without screen refresh on a My Block

Learning outcome

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

Say why a loop ran more times than you expected.
Make hundreds of clones and leave none behind.
Spot a game that has quietly run out of clones.

Learnt — thirty pictures a second 28 min

1 · The rule under everything


repeat until <(timer) > (7)>
  change [seen v] by (1)
  change y by (0)
end

Moves something. 181 turns in six seconds.


repeat until <(timer) > (7)>
  change [unseen v] by (1)
end

Moves nothing. Tens of thousands of turns in the same six seconds — 15,395 on one run, 90,434 on another.

WHAT IT SHOULD LOOK LIKE

the same six seconds

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch the two monitors race. Both loops stop at the same moment.

181 against tens of thousands. The 181 is the same every run; the other number never is, which is the point.

2 · Where a crowd comes from

Our engine makes one speck of dust per tick of thrust. That sounds tiny. A gentle descent is 180 ticks long and burns for about a third of them:


if <(costume [name v]) = [lander-thrust]> then
  set [puff x v] to (x position)
  set [puff y v] to ((y position) - (24))
  create clone of (Dust v)
end

The lander makes the clone, so no speck ever hears an order meant for the original — Lesson 3-37’s bug, avoided by not broadcasting.

WHAT IT SHOULD LOOK LIKE

One speck per tick of thrust. A gentle descent burns for about a third of its 178 ticks, so a single flight makes 56 of them.

one flight

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch the two monitors: specks made climbs all the way down, specks alive stays small.

56 specks in one flight, and four alive at the end — because each one deletes itself.

3 · Making them is not free either

create clone of (myself v) puts something new on the Stage, so it waits for the next drawing too. Two hundred clones in an ordinary loop take two hundred frames to arrive.

Runs here in the page. Nothing to install.

How to use this: Click the green flag. The same two hundred specks are made twice — first in an ordinary loop, then inside a ticked My Block.

One at a time: 6.6 seconds. All at once: 0 seconds. Same two hundred specks.

Basic — dust that tidies up 24 min

Give your own game something that appears in numbers — dust, sparks, bubbles, coins — and make sure every one of them leaves.

Step 1 — one clone per tick

Make the sprite and two variables, puff x and puff y.
In the flight loop, set those two and create clone of (Dust v).
Run it. The Stage should fill up quickly.

Step 2 — a life, then a goodbye

In when I start as a clone, go to the puff, then repeat (12) drifting and fading.
After the repeat, delete this clone. Not hide.
Run it again. The Stage should now stay tidy while the flight goes on.

Step 3 — fewer, and nobody notices

Make a speck only every third tick: if <((ticks) mod (3)) = (0)> then.
Watch it. It looks the same, and it is a third of the clones.

Runs here in the page. Nothing to install.

How to use this: Click the green flag and hold the up arrow to make dust. The two monitors count what was made and what is still alive.

What you are aiming at: a long burn, a trail of dust, and specks alive that never climbs.

Every speck appears in the same place.

The dust fades but the Stage still fills up.

You used hide or ghost 100 instead of delete this clone. Invisible is not gone — that is Challenge 1.

My specks alive monitor counts up and never down.

Put change [specks alive v] by (-1) directly above delete this clone. After the delete, nothing runs.

After a while no new dust appears at all.

You have hit three hundred. Everything else in your project has stopped being able to clone as well — see Challenge 2.

CHALLENGE 1 · L3-47-M1🪙 30 Adcoins8 min

The specks that never leave

This lander’s dust fades to nothing, which looks exactly like tidying up. Prove that it is not.

Watch both monitors for a whole flight.
Compare what was made with what is still alive at the end.
Work out how many flights this game would survive.

WHAT IT SHOULD LOOK LIKE

Challenge 1 — a speck fades to ghost 100 and stops being visible. It has not stopped being a clone: 56 made, 56 still alive.

faded, not gone

Runs here in the page. Nothing to install.

How to use this: Click the green flag and let it land. Read specks made and specks alive at the end — they are the same number.

56 made, 56 alive. Four more flights like that and the project is full.

Done when: you can say how many flights this project gets before it can make no more clones, and your own version keeps specks alive under twenty.

Teacher note

CHALLENGE 2 · L3-47-M2🪙 50 Adcoins9 min

The explosion that never came

Three hundred is the whole project’s allowance, not one sprite’s. Here is what that looks like from the inside.

The dust asks for four hundred specks. Count what it gets.
The crash then asks for five pieces of wreckage.
Find the error message Scratch shows. There is not one.

WHAT IT SHOULD LOOK LIKE

Challenge 2 — four hundred specks were asked for and three hundred arrived. Then the crash asked for five pieces of wreckage and got none at all.

the ceiling

Runs here in the page. Nothing to install.

How to use this: Click the green flag. Read specks alive first, then watch the bits monitor when the lander switches to its crashed costume.

400 asked for, 300 made. Then five pieces of wreckage were asked for, and none appeared.

Done when: you can explain why a sprite that has never made a clone in its life can still fail to make one.

Teacher note

CHALLENGE 3 · L3-47-M3🪙 70 Adcoins10 min

Hidden is not gone

Same three hundred specks. This time every single one of them is hidden, so the Stage is empty — and the project is just as full.

Run it. Look at the Stage, then look at the monitor.
Ask for the five pieces of wreckage again.
Say, in one sentence, the difference between hide and delete this clone.

WHAT IT SHOULD LOOK LIKE

Challenge 3 — the same three hundred, every one of them hidden. The Stage looks empty and the project still cannot make one more clone.

hidden is not gone

Runs here in the page. Nothing to install.

How to use this: Click the green flag. Nothing appears — read the specks alive monitor, then the bits monitor after the crash.

An empty Stage holding three hundred clones, and no room for five more.

Done when: you have found any hide in your own project that should have been a delete, and changed it.

Teacher reveal — the one-line difference


when I start as a clone
show
repeat (12)
  change y by (-2)
  change [ghost v] effect by (6)
end
change [specks alive v] by (-1)
delete this clone

hide changes what a clone looks like. delete this clone ends it: its scripts stop, and its place in the three hundred is given back. Only one of those two makes room for the next one.

EXTRA MISSION · L3-47-MX🪙 100 Adcoinsbonus

Run without screen refresh

A My Block has a tick-box nobody reads. Turning it on tells Scratch to finish the whole block before drawing anything — so two hundred steps happen inside one frame.

Make a My Block that does two hundred steps of work.
Time it with (timer), twice: tick off, then tick on.
Use it for something real — building a crowd, or checking a whole list.

Runs here in the page. Nothing to install.

How to use this: Click the green flag and wait. The same two hundred steps run twice, and the sentence at the end gives both times.

Ordinary: 6.6 seconds. Run without screen refresh: 0 seconds. Identical work, identical answer.

Teacher note

Good place to name the trade the whole lesson is about: Scratch spends its time either drawing or thinking. Every trick here is a way of choosing which.

Summary 5 min

The Stage is drawn about thirty times a second.
A loop that changed something visible waits for that. One that did not, does not.
A project may hold 300 clones at once, shared by every sprite.
hide is not delete this clone.
A ticked My Block finishes before the next drawing — good for work, not for movement.

Frame: One drawing of the Stage. About thirty of them a second.
Redraw: Scratch putting the Stage on the screen again after something changed.
Clone limit: The 300 clones a whole project may have alive at one time.
Run without screen refresh: A My Block setting that finishes all its work inside one frame.

Hand in

Save your game with a clone counter on the Stage. Next week it gets a score, a board, a front door and a proper test.