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CS Lesson 2
Lesson 2: Applying decomposition — Make a sandwich
Lesson 2: Applying decomposition — Make a sandwich
Learning intention: Break making a sandwich into smaller jobs, explain how their outputs connect, and improve instructions through testing.
Core knowledge for retrieval: Decomposition divides a complex problem into smaller problems with identifiable outputs. An algorithm gives precise instructions. Debugging involves finding and fixing errors; changes need to be tested again.
Resources and preparation: Paper sandwich props, or bread, jam, margarine, food-safe gloves, cutting boards and suitable spreading knives. Check dietary requirements and establish food-handling and equipment routines before starting. Have paper or devices available for recording instructions and test results.
Teacher preparation video: Exact instructions sandwich activity.
Be ready to answer!
Without looking at your notes:
Explain what decomposition means.
Name three smaller jobs involved in making a sandwich.
Identify two jobs that must happen in order. Explain why.
TASK 1: WATCH AND TRY · 15 MINUTES
Look at the ingredients and equipment, or paper props, provided.
With a partner, write numbered instructions for making a sandwich.
Include one clear action per step.
Watch your teacher follow a set of instructions exactly as written.
Record one missing detail or unexpected result.
Identify the output of one job and explain which job uses it next.
Remember: Decomposition identifies smaller jobs. An algorithm gives precise instructions for carrying them out.
TASK 2: FIND AND FIX A PROBLEM · 15 MINUTES
Work with your teacher to improve an instruction.
Identify a missing, unclear or incorrectly ordered step.
Explain the problem it caused.
Help rewrite the instruction.
Predict what will happen when it is followed.
Watch the test. Did the change work?
Use what you learned to improve your own sandwich instructions.
Complete: “We changed _____ because _____.”
Remember: Debugging means finding and fixing errors. Retesting checks whether the fix worked.
TASK 3: SWAP, TEST AND IMPROVE · 10 MINUTES
Join another pair. Use the paper props or approved ingredients, following your teacher’s food-handling and equipment instructions.
Group your instructions into smaller jobs, such as preparing the bread, adding the filling and assembling the sandwich.
Write the output of each job.
Swap instructions with the other pair.
Take turns reading, making and observing.
Follow the instructions as written. If a step is unclear, pause and record the problem.
Improve that instruction and test it again.
Record:
Original instruction → Problem observed → Improved instruction → Retest result
Discuss: Which job needs an output from an earlier job?
EXIT CHECK · 5 MINUTES
Complete these independently:
Decomposition helped us by…
One unclear instruction was…
We improved it by…
Our retest showed…
If asked to share, use a specific result from your test to support your explanation.
Hand in: Your original and improved instructions.
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