Mouse Down? Mouse X, Mouse Y

Learning Goals 3 min

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

1. Recognise the three mouse-sensing blocks in the Sensing palette and name them: <mouse down?> (boolean), (mouse x), and (mouse y) (round reporters).
2. Explain that (mouse x) and (mouse y) are live coordinates — they change every frame the mouse moves, and they use the same Stage range you already know: x: −240..+240, y: −180..+180.
3. Combine <mouse down?> with forever + if <> then + go to x: () y: () to build a sprite that follows the mouse only while the button is held down.

Warm-Up — where is the mouse, anyway? 7 min

The Stage has its own number map. The middle is (0, 0), the right edge is x: 240, the top edge is y: 180. You've used those coordinates for sprites since Level 1. But the mouse pointer also has coordinates — and Scratch will tell you them, live, if you ask.

when flag clicked
forever
  say (mouse x)
end
        

Imagine you hit the flag and then slide the mouse from the left edge of the Stage to the right edge. What numbers do you predict the cat will say?

Predict-this number two:

when flag clicked
forever
  if <mouse down?> then
    say [click!] for (0.2) seconds
  end
end
        

Today's lesson is about those three blocks — <mouse down?>, (mouse x), and (mouse y) — and the fact that together they let you turn the mouse into a steering wheel, a paintbrush, or a trigger.

New Concept — three blocks, two shapes 15 min

Scroll down in the light-blue Sensing palette. Past the touching blocks and the distance-to block from last lesson, you'll find three mouse blocks sitting together.

The boolean: <mouse down?>

This one is hexagonal — same shape as touching [edge v]?. It reports true whenever any mouse button is being held down, and false the rest of the time.

<mouse down?>
        

It does not tell you which button (Scratch doesn't distinguish left/right/middle). It does not tell you where. It only tells you "is a button down right now, this instant?".

The reporters: (mouse x) and (mouse y)

These two are round — same shape as x position. They report the current mouse coordinates on the Stage.

(mouse x) :: sensing
(mouse y) :: sensing
        

Same coordinate system as sprites: x goes from about -240 (left edge) to 240 (right edge), y goes from -180 (bottom) to 180 (top). The numbers update continuously — there is no "click required". The instant you move the mouse, the reporter changes.

The classic combo: follow-the-mouse

Drop both reporters into the matching round slots of go to x: () y: () and put the whole thing in a forever loop:

when flag clicked
forever
  go to x: (mouse x) y: (mouse y)
end
        

Gating it with <mouse down?>

Now wrap that follow-the-mouse block in an if-then with <mouse down?>. The sprite follows your pointer only while you're holding the button. Release, and it freezes. This is the heart of every drag-to-paint app:

when flag clicked
forever
  if <mouse down?> then
    go to x: (mouse x) y: (mouse y)
  end
end
        

Worked Example — Aisyah's come-here pet 12 min

We're going to make the cat-pet come when called: while the button is held, the pet glues to the pointer; release, and the pet stays where you let go.

Step 1 — New project, keep the cat-pet

New Scratch project. Keep the default cat — this is our pet. (If you painted your own pet earlier, use that.) Roughly 60×60 pixels is plenty.

Step 2 — Position the pet

Drag the pet on the Stage to roughly x: 0, y: 0. We want it visible in the middle so we can see it move.

Step 3 — Drop the hat

From Events: when ⚑ clicked.

Step 4 — Add the forever loop

From Control: forever. Snap it under the hat.

Step 5 — Add the if-then

From Control: if <> then. Drop it inside the forever.

Step 6 — Drop <mouse down?> into the diamond

From Sensing: <mouse down?>. Drag it onto the diamond slot. It snaps in.

Step 7 — Add the go-to inside the if-then

From Motion: go to x: () y: (). Drop it inside the if-then C. The two round slots are empty.

Step 8 — Fill the round slots with (mouse x) and (mouse y)

From Sensing: drag (mouse x) into the left round slot, then drag (mouse y) into the right round slot. Both snap in.

Click the flag. Click on (or near) the pet and drag — the pet follows the pointer. Let go — the pet freezes. Move the mouse without holding — the pet ignores you. You just built a come-here pet with seven blocks, no special "make draggable" tickbox required.

The full assembled stack

when flag clicked
forever
  if <mouse down?> then
    go to x: (mouse x) y: (mouse y)
  end
end
        

What you just built: the core of every paint program, every drag-and-drop puzzle, every "move the piece" board game on the Internet. The mouse is no longer just a clicker — it's a controller with coordinates.

Try It Yourself — three mouse drills 15 min

when flag clicked
forever
  go to x: (mouse x) y: (mouse y)
end
          

when flag clicked
forever
  if <mouse down?> then
    say (mouse y)
  else
    say (mouse x)
  end
end
          

when flag clicked
forever
  glide (0.3) secs to x: (mouse x) y: (mouse y)
end
          

Mini-Challenge — Daniel's frozen draw 5 min

"Why won't the line follow?"

Daniel wants to draw with the mouse using the Pen extension. He drops the Pen extension and writes:

when flag clicked
erase all
pen down
go to x: (mouse x) y: (mouse y)
forever
  if <mouse down?> then
    pen down
  else
    pen up
  end
end
        

Look closely at where the sprite actually goes during the forever loop.

when flag clicked
erase all
pen up
forever
  go to x: (mouse x) y: (mouse y)
  if <mouse down?> then
    pen down
  else
    pen up
  end
end
          

Recap 3 min

You met three new Sensing blocks that turn the mouse into an input device. <mouse down?> is hexagonal — it reports true while any mouse button is held. (mouse x) and (mouse y) are round — they report the pointer's live Stage coordinates, in the same -240..+240 / -180..+180 range you use for sprites. Drop them into go to x: () y: () for follow-the-mouse, gate that with the boolean for click-to-drag, or save them into variables to remember a click position. Every mouse-based game and paint app rests on these three blocks.

Boolean reporter

A hexagonal block that reports true or false. <mouse down?> is one — true while a mouse button is held, false otherwise.

Round reporter

An oval block that reports a value (usually a number). (mouse x) and (mouse y) are round; they fit into any round slot.

Live value

A reporter whose answer changes on its own as the world changes. The mouse reporters are live — you don't read them, they are the current truth, every frame.

Stage coordinates

The Scratch coordinate system. x from -240 (left) to 240 (right). y from -180 (bottom) to 180 (top). The mouse uses exactly this system.

Drag-to-paint

The classic combo of <mouse down?> + go to x: (mouse x) y: (mouse y) inside a forever — the heart of every paint, draw, or drag-puzzle app.

Homework 2 min

Mouse-Brush Mini-App. Build a tiny paint program using only what you know.

1. New project. Delete the cat. Add a small round sprite (a dot, a circle — paint your own or pick from the library) and add the Pen extension from the bottom-left button.
2. Top of the script: when ⚑ clicked → erase all → pen up.
3. Then a forever. Inside the loop: go to x: (mouse x) y: (mouse y), then an if <mouse down?> then else with pen down in the yes branch and pen up in the no branch.
4. Add a second sprite — a tiny "C" button. Give it a when this sprite clicked → erase all script so clicking it wipes the canvas.

Save as HW-L2-16-Mouse-Brush.sb3. Click the flag and draw your name, a kuih, or a teh tarik glass with milky lines. Use the C button to clear.

Bring back next class:

• The .sb3 file.
• Your answer to: "What happens if you swap (mouse x) and (mouse y) in the go-to block? Try it. Why does it feel so wrong?"

Heads up for next class: SCR-L02-17 does for the keyboard what today did for the mouse — meeting when [space v] key pressed (the hat block, fires once) and <key [space v] pressed?> (the boolean, true while held). Two ways, one keyboard.