change x by 20move right
Public lesson / Scratch & Game Design
How Does the Screen Know Where You Are?
A giant pigeon is chasing your character across Calgary. The computer cannot see the chase the way a person does. How does it know where the player is, where the pigeon is, or whether they have collided?
Lesson rhythm
The idea moves toward a demonstration.
Problem and story
Turn a ridiculous chase into a precise position problem.
Coordinates and movement
Read x and y, predict changes, and reset to the origin.
Guided build
Build the core movement and chase systems together.
Modification challenge
Change one rule and test the consequences.
Demonstration and explanation
Show the system and explain its numbers.
Learning goals
What students should be able to do and explain.
- 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.
Theory kernel
Small examples before the larger build.
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-build preview
Escape from the Giant Pigeon
Reach a safe place while a huge, unreasonable pigeon chases you across the map. The game should be funny to play and precise enough to explain.
Keyboard movement in four directions, Known starting positions, A pursuing pigeon, One or more safe areas.
Coordinates, Movement, Conditions, Collision, Game State.
A working version, one modification, one repaired problem, and an explanation.
Teacher checkpoints
Evidence before adding complexity.
- 1Student can identify the centre.
- 2Student can predict four movements before running them.
- 3Student can reset the player to a known position.
- 4Student can explain one bug using x or y.
- 5Student has changed at least one rule independently.
Common mistakes
Pause, inspect, and test one cause.
- 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.
Modification challenge
Three levels, one concept kept visible.
Higher levels are optional. They deepen the current idea rather than turning a beginner lesson into an unrelated advanced task.
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
Explain the work while it is still visible.
- What did you build?
- Which number controls left and right?
- Which number controls up and down?
- What went wrong?
- What did you change?