Session 7 — The X–Y Grid

Duration: 60 min · Format: live online

Session goal: by the end, students can read the stage as a coordinate grid — x across, y up/down, centre at x:0 y:0 — and place a sprite at an exact spot with go to x: y:, glide it smoothly with glide … secs to x: y:, and nudge it with change x by and set x to.

Soft skill focus — Curiosity

Today's human skill: Curiosity. The x–y grid rewards students who keep asking "what happens if I try this number?" as they hunt for exact spots.

• Build it in class: during the coordinate treasure hunt, invite students to guess an address before they type it, then run it to see how close they got — celebrate the wondering, not just the right answer.
• Reflection (wrap-up): ask "What is one thing about the grid you wanted to try out and test today?"

Before class — prep

• Open Scratch in a tab (signed in if you can, so you can save) and be ready to screen-share the editor.
• Have the x–y grid diagram below open to share, and know the four corner numbers by heart: top-right is about x:240 y:180, bottom-left about x:−240 y:−180.
• Turn on the mouse-position readout: as you move the mouse over the stage, Scratch shows the x and y under the pointer at the bottom-right of the stage — you'll point at this all lesson.
• If any student can't run Scratch, have the low-tech fallback (a paper grid) ready — see Teaching notes.

Hook

Ask the class (chat or unmute):

• "If I told a friend to 'meet me at the park', that's fuzzy. How could you tell them the exact spot so you land in the same place every time?"

Take three answers (a lot will say "give directions" or "use a map"). Then reveal: "Computers hate 'over there'. The Scratch stage has a hidden map of numbers — every single point has an address. Today you learn that address language, and you'll be able to send your sprite to any exact spot on the screen."

Teach — The stage is a grid: x, y and the centre

Share this diagram and trace the two arrows with your pointer as you talk:

Explain, slowly, pointing on your shared screen:

• x is the across number. It counts left to right. The far left edge is about −240, the far right edge is about 240. Bigger x = further right; negative x = left.
• y is the up-and-down number. It counts bottom to top. The bottom edge is about −180, the top edge is about 180. Bigger y = further up; negative y = down.
• The centre is x:0 y:0. That's the exact middle of the stage. Every point on the screen has its own pair of numbers — an address — written x first, then y.

Now make it real. Slowly slide your mouse across the stage and read the x and y readout at the bottom-right aloud: "watch — as I go right the x number grows… as I go up the y number grows… right in the middle it's near zero, zero."

Ask: "To go to the top-right corner, do I want x and y to be big-positive or big-negative?" (Answer: both big-positive — like x:240 y:180.) "And the bottom-left?" (Both big-negative — x:−240 y:−180.)

⚠ Watch for the mix-up: x and y get swapped constantly. Keep repeating the rule: x = aCROSS, y = up (the tall letter y stands up). Always say the numbers in order: x first, then y.

Teach — go to, glide, change x, set x

Explain: now that the stage has addresses, Motion gives you blocks that use them. Build these live on your shared screen, running each one so the class sees the sprite jump or glide.

Build this together:

1. From Motion (blue), drag out go to x: 0 y: 0 and click it — the sprite snaps instantly to the centre. Change it to go to x: 200 y: 0 and click — it teleports to the right. go to is instant, like blinking to a spot.

2. Swap in glide 1 secs to x: -200 y: 0 and click — this time the sprite slides smoothly across. glide is the same idea but it takes time, so you see it travel. Change the seconds to 2 to make it slower.

3. From Motion, try change x by 50 and click it a few times — the sprite steps to the right 50 each click, without touching y. change x by means "add to where you already are."

4. Now set x to 0 and click — it jumps to the middle line (x:0) but keeps its height. set means "make it exactly this," change means "add on."

Put it together into one short script:

when green flag clicked
go to x: 0 y: 0
glide 1 secs to x: 200 y: 100
change x by -100

Press the green flag. The sprite starts in the middle, glides up to the right, then hops back left. Ask the class to predict the last position before you show it.

Optional taste of Pen — draw a line. If time allows, add the Pen extension (bottom-left Add Extension button → Pen). Build: go to x: -150 y: 0 → pen down → glide 1 secs to x: 150 y: 0 → pen up. Run it — the sprite draws a straight line across the stage as it glides. The pen leaves a trail wherever the sprite moves.

Ask: "What's the difference between go to and glide?" (Go to = instant/teleport; glide = smooth over time.) "And set x to vs change x by?" (Set = exact value; change = add on to what's there.)

Activity — Send your sprite on a coordinate treasure hunt

Students place and move a sprite by its address. Demo the first step, then let them go and circulate.

1. Open a new project at Scratch. Move your mouse over the stage and watch the x/y readout at the bottom-right.
2. Build when green flag clicked → go to x: 0 y: 0 so your sprite always starts in the centre. Press the flag.
3. Hit four corners: add four blocks in a row — glide 1 secs to x: 200 y: 150, then … x: -200 y: 150, then … x: -200 y: -150, then … x: 200 y: -150. Run it — the sprite glides around the edge of the stage like a picture frame.
4. Make it yours: add change x by 50 and click it to walk the sprite sideways, and set y to 0 to snap it back to the middle line. Try making the sprite spell a shape by gliding between spots.
5. Extra (Pen): add the Pen extension, put pen down after your first go to, and let the glides draw the frame as the sprite travels. pen up at the end.

Circulate and ask: "What address is your sprite at right now?" "Which number moves it up — x or y?"

Debrief: ask 2–3 students to share their screen and call out the address their sprite lands on. Cheer the ones who drew a shape with the pen.

Check for understanding

Ask these aloud or in the chat. Answer key (for you):

1. What does the x number control, and what does y control? → x moves the sprite across (left–right); y moves it up and down.
2. What is the address of the exact centre of the stage? → x:0 y:0.
3. What's the difference between go to x: y: and glide … secs to x: y:? → go to teleports the sprite instantly; glide moves it smoothly over time so you see it travel.

Wrap-up + homework

• Ask one student to explain, in one sentence, the rule for remembering x and y (x is across, y is up).
• Homework — Four-corner tour: make your sprite start at x:0 y:0, then glide to visit all four corners of the stage and return to the centre. Screenshot the script for your portfolio.

Teaching notes

• Correct this misconception: "bigger numbers always mean 'more to the right / higher up'." True for x and y separately, but students forget the numbers can be negative — left and down are the negative side. Keep pointing at the readout while you drag left so they see x go below zero.
• Fast finishers (extension): challenge them to make the sprite draw a square with the Pen using four glides that return to the start, then a triangle with three. Ask them to work out the corner addresses on paper before they build.
• Low-tech fallback: no devices? Draw a big grid on the board (or hand out squared paper) with 0,0 in the middle and label the edges −240/240 and −180/180. Call out addresses ("go to x:100 y:−50!") and have students place a counter or point to the spot. The idea — every point has an x,y address — is the real lesson; Scratch is just the tool.

Vocabulary

Resources

• Scratch editor — where students build (free, in the browser).
• Motion blocks reference (Scratch Wiki) — every Motion block explained, including go to x: y: and glide.
• Scratch Ideas page — official tutorials and starter cards.

Practice set

Extra tasks to explore the x–y grid. Work them easy → hard, at lab time or for homework. Answers follow each arrow.

1. Read the address. A sprite is at the exact middle of the stage. What is its x and y? → x:0 y:0.

2. Which number? You want your sprite to move up. Do you change x or y? → y (the up–down number).

3. Corner clue. Which corner is roughly at x:−240 y:180? → The top-left corner (far left, near the top).

4. Predict. The script is go to x: 0 y: 0 → change x by 100. Where is the sprite now? → At x:100 y:0 — 100 to the right of centre, same height.

5. Make it (build task). Build a script so the sprite starts in the centre and glides to the top-right corner over 2 seconds. → when green flag clicked → go to x: 0 y: 0 → glide 2 secs to x: 200 y: 150.

6. Set vs change (harder). The sprite is at x:150 y:0. You run set x to 0. Then you run change x by 20. What is its x now? → 20. set x to 0 makes x exactly 0; change x by 20 then adds 20.

Going deeper (optional)

Optional — for a class that can place a sprite by its address and wants to know why coordinates matter.

This is the same map used everywhere. The x–y grid isn't a Scratch invention — it's how maps, graphs and every video game describe positions. When a game knows a player is at "x:340, y:88," it's using the exact idea your students just learned. Mention that in Session 9 they'll use y to make apples fall down the screen — by making y get smaller and smaller.

Why is 0,0 in the middle, not the corner? In Scratch the centre is 0,0 so a sprite has room to go positive and negative in every direction — handy for symmetrical movement (glide to x:100, then x:-100, same distance each side). Other tools (and image files) often put 0,0 in a corner. There's no single "right" place for the origin — it's a choice the tool's makers make.

Common mistakes & fixes

• Mistake: Swapping x and y, so the sprite goes sideways when they wanted up. → Fix: repeat the rule x is across, y is up, and always read the address x first, then y.
• Mistake: Expecting change x by 50 to set the position, then confused it "keeps adding." → Fix: change adds on each time; use set x to when you want an exact spot.
• Mistake: Typing a huge number like x:900 and the sprite "vanishes." → Fix: it glided off the edge — the stage only reaches about ±240 across and ±180 up/down; use go to x:0 y:0 to bring it back.
• Mistake: The Pen draws nothing. → Fix: you need pen down before the sprite moves; check the Pen extension was added from the bottom-left button.
• Mistake: Forgetting the sprite starts wherever it was left, so scripts "work differently each run." → Fix: begin the script with go to x:0 y:0 (or any fixed spot) so it always starts from the same address.

Next session

Session 8 — Ask, Answer & Build a Quiz: students make their project talk back — the sprite asks a question with ask [ ] and wait, reads the player's typed answer, checks it with an if block, and keeps score to build a real 3-question quiz.