change x by 20move right
Lesson / Scratch & Game Design
How Does the Screen Know Where You Are?
A giant pigeon is chasing your character across Calgary. Give both sprites reliable positions, move the player with x and y, and decide exactly what counts as being caught.
Lesson plan
The chase
Work out how the stage records the player and pigeon positions.
Coordinates
Read x and y, predict changes, and return to the origin.
Build the game
Add movement, pursuit, collision, and a safe zone.
Change a rule
Choose one modification, predict the result, and try it.
Show and explain
Run the game and use coordinates to explain one part.
Learning goals
- Read x and y positions.
- Move a sprite horizontally and vertically.
- Set a known starting position.
- Predict the result of coordinate changes.
- Recognize screen boundaries.
- Use position information while debugging.
- Explain movement using numbers.
Small examples
change x by -20move left
change y by 20move up
change y by -20move down
go to x: 0 y: 0return to the centre
Scratch direction and Scratch position are related, but they are different. Direction describes where a sprite faces; position describes where it is.
Guided project
Escape from the Giant Pigeon
Reach the shelter before a huge pigeon catches you. Under the absurd premise is a precise system of coordinates, collision checks, and two possible endings.
Keyboard movement in four directions, Known starting positions, A pursuing pigeon, One or more safe areas.
Scratch, Coordinates, Movement, Collision, Game State.
A working version with a tested change, one repaired problem, and a clear explanation.
Teacher checkpoints
- 1The student can identify the origin.
- 2Four coordinate changes are predicted before they run.
- 3Restarting returns the player to the chosen position.
- 4The student explains one movement bug using x or y.
- 5At least one rule has been changed independently.
Common mistakes
- Changing x when the intended movement is vertical.
- Using the wrong positive or negative sign.
- Testing from an unknown starting position.
- Confusing the direction a sprite faces with its position.
Optional challenges
Builder
- Add screen wrapping.
- Add a second safe zone.
Inventor
- Create a temporary turbo move.
- Add a boundary warning.
Boss Level
- Optionally make pigeon speed depend on approximate distance.
End-of-class demonstration
- What did you build?
- Which number controls left and right?
- Which number controls up and down?
- What went wrong?
- What did you change?