Launch Week II — Scratch L3 Lesson 46

Learnt28 min · expect a number, then go and look
Basic24 min · mend it yourself
Challenge 1–327 min · a leak, a lie, and a second go
Extrabonus · the countdown nobody saw

WHAT IT SHOULD LOOK LIKE

Same rocket, same moon, same code you wrote last week — and it is wrong in five different ways. Today you find them, one number at a time.

something is pulling twice

Review — the slice 5 min

A design sheet is seven lines, and one of them is a single tick.
Falling is a number you keep, not a glide.
One My Block decides what happened; one variable holds it.

Quick-fire

Your lander reaches the ground too quickly. Where do you look first?

Reveal the answer

At a number, not at the blocks. “Too quickly” is a feeling. “60 ticks in the plan, 42 on the screen” is a fact, and a fact tells you which half of the code to open.

Today’s Topic 3 min

Saying what you expect, in numbers, before you look
Watching the Stage instead of the script — monitors and say (join [fall is ] (fall))
Halving the problem until only one script is left
Changing one thing at a time, and checking after each

Learning outcome

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

Turn “it feels wrong” into two numbers that disagree.
Find which script is lying by switching half of them off.
Mend four bugs in a project you did not write.

Learnt — four moves 28 min

1 · A bug is a disagreement

A bug is not “something broken”. It is your plan and your program saying different things. So the first move is always the same: write down the number you expect. Last week’s flight took 60 ticks and arrived at 7.5. That is the plan.

WHAT IT SHOULD LOOK LIKE

Two flights from the same height. The plan’s lander takes 60 ticks and arrives at 7.5; this one takes 42 and arrives at 10.5.

the plan’s flight

this flight

panels run left to right

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch the ticks monitor. It is the same flight as last week, from the same height.

42 ticks, arriving at 10.5. Nothing looks broken. The lander is simply twice as heavy as the plan says.

2 · Make the numbers say themselves

You cannot read a running loop. You can make it talk. Drag fall onto the Stage, and put one say inside the loop:


if <((ticks) mod (10)) = (0)> then
  say (join (join [tick ] (ticks)) (join [: fall is ] (fall))) for (0.6) seconds
end

mod keeps it to every tenth tick, so the bubbles are readable instead of a blur.

WHAT IT SHOULD LOOK LIKE

The fix for “it feels wrong” is a number beside a number. Every tenth tick the lander says what fall is, next to what the plan says it should be.

reading it out loud

Runs here in the page. Nothing to install.

How to use this: Click the green flag and read the bubbles. Each one shows fall beside the number the plan expects.

Twice the plan on every tick, not just at the end. A doubled block, not bad luck.

That last sentence is the whole move. Twice, every tick rules out a wrong starting value and rules out the ground. It points at gravity itself.

3 · Halve the problem

When a project is too big to read, do not read it. Switch off half of it and ask the question again.

Drag half the scripts out of the way, into the empty grey area.
Run it. Is the bug still there?
Still there? The bug is in the half you kept. Gone? It is in the half you dragged out.
Put the half back, and halve the guilty half again.

Four rounds of that narrows sixteen scripts down to one. It feels slow and it is the fastest thing you can do.

4 · Change one thing

When you find it, change one block and run it again. Two changes at once and you will never know which one worked — or which one broke something else.

Runs here in the page. Nothing to install.

How to use this: Click the green flag. One block has been deleted since the first embed; nothing else was touched.

60 ticks, speed 7.5. The plan’s own numbers, back again.

Basic — mend it yourself 24 min

Open your own lander from last week and put the doubled gravity in on purpose. Then find it, using the four moves and nothing else.

Step 1 — break it, and write the number down

Duplicate change [fall v] by (-0.125) inside the loop.
Before running it, write on paper: how many ticks do you expect?
Run it. Write down what actually happened, next to your guess.

Step 2 — make it talk

Drag fall and ticks onto the Stage.
Add the every-tenth-tick say from Learnt part 2.
Run it and read three bubbles. Is it wrong by a little, or wrong by double?

Step 3 — halve, fix, check

Drag every script except the flight loop out of the way. Bug still there?
Find the doubled block and delete one of the pair.
Run it. Your number should now match the paper.
Take the say back out — a mended project should be quiet.

My say bubble flickers and I cannot read it.

It is running every tick, thirty times a second. Wrap it in if <((ticks) mod (10)) = (0)> then and give it for (0.6) seconds.

The numbers look right but the game still feels wrong.

Then your plan is the thing that is wrong, not your code. That is allowed, and it is worth saying out loud: change the sheet, then change the blocks to match it.

I fixed it, and now something else is broken.

You changed more than one thing. Undo with Ctrl + Z until the game is back to one bug, and start again, one block at a time.

I cannot drag a script out of the way — it is stuck to the others.

You are dragging from the middle of a stack, which takes everything below it. Drag the hat block at the very top instead, and the whole script comes as one.

CHALLENGE 1 · L3-46-M1🪙 30 Adcoins8 min

The tank that leaks

This lander burns fuel it never used. Find out where it goes — the gauge is the only witness.

Watch the fuel monitor beside the flame.
Count roughly how many ticks the engine is actually lit.
Compare that with how much fuel disappeared.

WHAT IT SHOULD LOOK LIKE

Challenge 1 — the tank empties whether or not the engine fires, so a flight that burned for a quarter of its ticks still arrives on an empty, and then negative, gauge.

the leak

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch the fuel monitor, not the rocket. Note when the flame is on and when it is not.

The engine fired 36 times in 146 ticks, and the whole tank went anyway — to minus 26.

Done when: you can say in one sentence where the fuel went, and your mended version ends with fuel left over.

Teacher reveal — one block, one step too high


define hold (speed)
change [fuel v] by (-1)
if <<(fall) < ((speed) * (-1))> and <(fuel) > (0)>> then
  change [fall v] by (0.375)
  switch costume to (lander-thrust v)
else
  switch costume to (lander v)
end

The spend is above the if, so it happens on every tick of the flight. Move it inside, next to the thrust it pays for. Note that it can also go below zero — a gauge that reads −26 is the bug telling you about itself.

CHALLENGE 2 · L3-46-M2🪙 50 Adcoins9 min

The landing that was a crash

This one is worse than a crash: it is a game that lies to you. It says landed after an arrival nobody could survive.

Let it fall with no engine at all and read the speed at the bottom.
Compare it with the 2.5 on your design sheet.
Find the question the game forgot to ask.

WHAT IT SHOULD LOOK LIKE

Challenge 2 — the test asks where, never how fast. The wreck is on the pad, so the game calls it a landing.

“landed”

Runs here in the page. Nothing to install.

How to use this: Click the green flag. Nobody touches a key — watch the fall monitor at the moment it stops.

Arriving at 7.6 and calling it a landing. The test asks where, and never how fast.

Done when: a fast arrival says crashed, a slow one says landed, and you can show both without changing anything but the keys you press.

Teacher note

This is the most valuable bug on the page, because nothing about it looks wrong. The project runs, the wording is sensible, and the test is even half right. Ask: “how would a player ever report this?” The answer — “the game said I won and I did not” — is what a real bug report sounds like.

CHALLENGE 3 · L3-46-M3🪙 70 Adcoins10 min

Fine once, broken twice

The first flight is perfect. The second one never starts — the lander sits at the top of the Stage doing nothing at all.

Watch state and fuel across both goes.
Ask what the second go inherited from the first.
List everything a new flight must put back.

WHAT IT SHOULD LOOK LIKE

Challenge 3 — go two never happens. The state still says crashed from go one, so the loop finishes before it starts.

go 1

go 2

panels run left to right

Runs here in the page. Nothing to install.

How to use this: Click the green flag once and watch both flights. The go monitor says which one you are on.

Go two began on go one’s leftovers, so the loop’s until was already true and it never ran.

Done when: three flights in a row all behave like the first one.

Teacher reveal — a new flight is a My Block


define new flight
go to x: (-60) y: (130)
switch costume to (lander v)
set [fall v] to (0)
set [fuel v] to (120)
set [state v] to [flying]
set [ticks v] to (0)

Every variable that changes during a flight has to be named here. The cure for “I forgot one” is to put them all in one block, so there is exactly one place to look — the same trick Lesson 3-43 used for a new round.

EXTRA MISSION · L3-46-MX🪙 100 Adcoinsbonus

The countdown nobody saw

A lift-off countdown was built by counting loop turns instead of seconds. On the screen, nothing counted at all.

Run it and watch the turns monitor.
Work out how many pictures were drawn while it counted.
Rebuild the countdown so a person can read it.

WHAT IT SHOULD LOOK LIKE

Extra — a countdown counted in loop turns. A hundred thousand of them went by in 1.06 seconds, while the Stage was drawn about thirty times.

the countdown nobody saw

Runs here in the page. Nothing to install.

How to use this: Click the green flag and keep your eye on the turns monitor. Try to see it move.

A hundred thousand turns in 1.06 seconds. A loop that changes nothing you can see never waits for the picture.

Teacher note

Worth timing on the board. Thirty drawings a second is the budget every Scratch game lives inside, and next week is about spending it well.

Summary 5 min

A bug is your plan and your program disagreeing about a number.
Write the number you expect before you look at anything.
Monitors and one say beat reading the script.
Halve the project until one script is left, then change one block.
Counting loop turns is not counting time.

Bug: A difference between what a program should do and what it does.
Reproduce: To make the same bug happen again, on purpose.
Halving: Switching off half the code to find which half holds the fault.
Reset: Putting every variable back before a new go begins.

Hand in

Save your mended game. Write one line at the bottom of your design sheet: the bug that took you longest, and the move that found it.

Learnt28 min · expect a number, then go and look
Basic24 min · mend it yourself
Challenge 1–327 min · a leak, a lie, and a second go
Extrabonus · the countdown nobody saw

WHAT IT SHOULD LOOK LIKE

Same rocket, same moon, same code you wrote last week — and it is wrong in five different ways. Today you find them, one number at a time.

something is pulling twice

Review — the slice 5 min

A design sheet is seven lines, and one of them is a single tick.
Falling is a number you keep, not a glide.
One My Block decides what happened; one variable holds it.

Quick-fire

Your lander reaches the ground too quickly. Where do you look first?

Reveal the answer

At a number, not at the blocks. “Too quickly” is a feeling. “60 ticks in the plan, 42 on the screen” is a fact, and a fact tells you which half of the code to open.

Today’s Topic 3 min

Saying what you expect, in numbers, before you look
Watching the Stage instead of the script — monitors and say (join [fall is ] (fall))
Halving the problem until only one script is left
Changing one thing at a time, and checking after each

Learning outcome

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

Turn “it feels wrong” into two numbers that disagree.
Find which script is lying by switching half of them off.
Mend four bugs in a project you did not write.

Learnt — four moves 28 min

1 · A bug is a disagreement

WHAT IT SHOULD LOOK LIKE

Two flights from the same height. The plan’s lander takes 60 ticks and arrives at 7.5; this one takes 42 and arrives at 10.5.

the plan’s flight

this flight

panels run left to right

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch the ticks monitor. It is the same flight as last week, from the same height.

42 ticks, arriving at 10.5. Nothing looks broken. The lander is simply twice as heavy as the plan says.

2 · Make the numbers say themselves

You cannot read a running loop. You can make it talk. Drag fall onto the Stage, and put one say inside the loop:


if <((ticks) mod (10)) = (0)> then
  say (join (join [tick ] (ticks)) (join [: fall is ] (fall))) for (0.6) seconds
end

mod keeps it to every tenth tick, so the bubbles are readable instead of a blur.

WHAT IT SHOULD LOOK LIKE

The fix for “it feels wrong” is a number beside a number. Every tenth tick the lander says what fall is, next to what the plan says it should be.

reading it out loud

Runs here in the page. Nothing to install.

How to use this: Click the green flag and read the bubbles. Each one shows fall beside the number the plan expects.

Twice the plan on every tick, not just at the end. A doubled block, not bad luck.

That last sentence is the whole move. Twice, every tick rules out a wrong starting value and rules out the ground. It points at gravity itself.

3 · Halve the problem

When a project is too big to read, do not read it. Switch off half of it and ask the question again.

Drag half the scripts out of the way, into the empty grey area.
Run it. Is the bug still there?
Still there? The bug is in the half you kept. Gone? It is in the half you dragged out.
Put the half back, and halve the guilty half again.

Four rounds of that narrows sixteen scripts down to one. It feels slow and it is the fastest thing you can do.

4 · Change one thing

When you find it, change one block and run it again. Two changes at once and you will never know which one worked — or which one broke something else.

Runs here in the page. Nothing to install.

How to use this: Click the green flag. One block has been deleted since the first embed; nothing else was touched.

60 ticks, speed 7.5. The plan’s own numbers, back again.

Basic — mend it yourself 24 min

Open your own lander from last week and put the doubled gravity in on purpose. Then find it, using the four moves and nothing else.

Step 1 — break it, and write the number down

Duplicate change [fall v] by (-0.125) inside the loop.
Before running it, write on paper: how many ticks do you expect?
Run it. Write down what actually happened, next to your guess.

Step 2 — make it talk

Drag fall and ticks onto the Stage.
Add the every-tenth-tick say from Learnt part 2.
Run it and read three bubbles. Is it wrong by a little, or wrong by double?

Step 3 — halve, fix, check

Drag every script except the flight loop out of the way. Bug still there?
Find the doubled block and delete one of the pair.
Run it. Your number should now match the paper.
Take the say back out — a mended project should be quiet.

My say bubble flickers and I cannot read it.

It is running every tick, thirty times a second. Wrap it in if <((ticks) mod (10)) = (0)> then and give it for (0.6) seconds.

The numbers look right but the game still feels wrong.

Then your plan is the thing that is wrong, not your code. That is allowed, and it is worth saying out loud: change the sheet, then change the blocks to match it.

I fixed it, and now something else is broken.

You changed more than one thing. Undo with Ctrl + Z until the game is back to one bug, and start again, one block at a time.

I cannot drag a script out of the way — it is stuck to the others.

You are dragging from the middle of a stack, which takes everything below it. Drag the hat block at the very top instead, and the whole script comes as one.

CHALLENGE 1 · L3-46-M1🪙 30 Adcoins8 min

The tank that leaks

This lander burns fuel it never used. Find out where it goes — the gauge is the only witness.

Watch the fuel monitor beside the flame.
Count roughly how many ticks the engine is actually lit.
Compare that with how much fuel disappeared.

WHAT IT SHOULD LOOK LIKE

Challenge 1 — the tank empties whether or not the engine fires, so a flight that burned for a quarter of its ticks still arrives on an empty, and then negative, gauge.

the leak

Runs here in the page. Nothing to install.

How to use this: Click the green flag and watch the fuel monitor, not the rocket. Note when the flame is on and when it is not.

The engine fired 36 times in 146 ticks, and the whole tank went anyway — to minus 26.

Done when: you can say in one sentence where the fuel went, and your mended version ends with fuel left over.

Teacher reveal — one block, one step too high


define hold (speed)
change [fuel v] by (-1)
if <<(fall) < ((speed) * (-1))> and <(fuel) > (0)>> then
  change [fall v] by (0.375)
  switch costume to (lander-thrust v)
else
  switch costume to (lander v)
end

CHALLENGE 2 · L3-46-M2🪙 50 Adcoins9 min

The landing that was a crash

This one is worse than a crash: it is a game that lies to you. It says landed after an arrival nobody could survive.

Let it fall with no engine at all and read the speed at the bottom.
Compare it with the 2.5 on your design sheet.
Find the question the game forgot to ask.

WHAT IT SHOULD LOOK LIKE

Challenge 2 — the test asks where, never how fast. The wreck is on the pad, so the game calls it a landing.

“landed”

Runs here in the page. Nothing to install.

How to use this: Click the green flag. Nobody touches a key — watch the fall monitor at the moment it stops.

Arriving at 7.6 and calling it a landing. The test asks where, and never how fast.

Done when: a fast arrival says crashed, a slow one says landed, and you can show both without changing anything but the keys you press.

Teacher note

CHALLENGE 3 · L3-46-M3🪙 70 Adcoins10 min

Fine once, broken twice

The first flight is perfect. The second one never starts — the lander sits at the top of the Stage doing nothing at all.

Watch state and fuel across both goes.
Ask what the second go inherited from the first.
List everything a new flight must put back.

WHAT IT SHOULD LOOK LIKE

Challenge 3 — go two never happens. The state still says crashed from go one, so the loop finishes before it starts.

go 1

go 2

panels run left to right

Runs here in the page. Nothing to install.

How to use this: Click the green flag once and watch both flights. The go monitor says which one you are on.

Go two began on go one’s leftovers, so the loop’s until was already true and it never ran.

Done when: three flights in a row all behave like the first one.

Teacher reveal — a new flight is a My Block


define new flight
go to x: (-60) y: (130)
switch costume to (lander v)
set [fall v] to (0)
set [fuel v] to (120)
set [state v] to [flying]
set [ticks v] to (0)

EXTRA MISSION · L3-46-MX🪙 100 Adcoinsbonus

The countdown nobody saw

A lift-off countdown was built by counting loop turns instead of seconds. On the screen, nothing counted at all.

Run it and watch the turns monitor.
Work out how many pictures were drawn while it counted.
Rebuild the countdown so a person can read it.

WHAT IT SHOULD LOOK LIKE

Extra — a countdown counted in loop turns. A hundred thousand of them went by in 1.06 seconds, while the Stage was drawn about thirty times.

the countdown nobody saw

Runs here in the page. Nothing to install.

How to use this: Click the green flag and keep your eye on the turns monitor. Try to see it move.

A hundred thousand turns in 1.06 seconds. A loop that changes nothing you can see never waits for the picture.

Teacher note

Worth timing on the board. Thirty drawings a second is the budget every Scratch game lives inside, and next week is about spending it well.

Summary 5 min

A bug is your plan and your program disagreeing about a number.
Write the number you expect before you look at anything.
Monitors and one say beat reading the script.
Halve the project until one script is left, then change one block.
Counting loop turns is not counting time.

Bug: A difference between what a program should do and what it does.
Reproduce: To make the same bug happen again, on purpose.
Halving: Switching off half the code to find which half holds the fault.
Reset: Putting every variable back before a new go begins.

Hand in

Save your mended game. Write one line at the bottom of your design sheet: the bug that took you longest, and the move that found it.