Keep this light. No materials out yet. Point at one everyday Irish example pupils know (traffic lights changing in order, a school-dinner queue, the PE warm-up sequence) and ask what would go wrong if a step vanished.

Key question: If the steps are wrong, does trying harder still get the right result? Steer them toward: no — the steps themselves must be right.

Link gently to inputs, processes and outputs from earlier work in this unit: an algorithm is the process that turns an input into an output.

Lead with the live plant model first. Name only algorithm and bug on the board at the start. Name debugging at the moment you fix the deliberate mistake. Treat abstraction as a light spoken phrase only ("keeping only what matters" / "crossing out the jumper detail") during the keep-or-leave-out talk — do not add it as a fourth word pupils must own on the board.

Worked example (model the full cycle aloud)

Do this live before groups write their own. Prefer the real plant, watering can and tray from the materials list. If those are unavailable, write the steps on the board and mime each action with an empty cup over a tray — the missing-step bug still lands clearly.

• I wonder: Can I write steps so precise that a classmate who has never watered our plant could do it without asking questions?
• I predict: If I leave out "walk to the plant", the water will go on the floor.
• I write (algorithm): 1. Pick up the watering can. 2. Check there is water in it. 3. Walk to the plant on the windowsill. 4. Pour slowly onto the soil, not the leaves. 5. Stop when the soil looks damp. 6. Put the can back on the tray. Say: "These ordered steps are an algorithm."
• Keep only what matters (say lightly, do not board the word abstraction): Cross out "smile at your friend" and "wear your favourite jumper" — those do not change the watering. Keep "onto the soil, not the leaves" — that does. Say: "Crossing out the jumper detail is keeping only what matters."
• I test: Ask a volunteer to follow the steps exactly while you watch. They should reach the plant and pour on the soil.
• Deliberate bug: Cover step 3 so it disappears. The volunteer pours where they stand. I observed water on the floor (or tray). Say: "That missing step was a bug." Put "walk to the plant" back, test again, and say: "Finding it, fixing it, and testing again is debugging."

Misconception to head off: pupils often think a bug means the person following the steps was careless. Stress that a careful person still fails if the algorithm is wrong. That is why we debug the steps, not blame the follower.

Nature of STEM: algorithms already run Irish daily life — traffic-light sequences, oven timers, and the order a bus-ticket machine follows when you tap a Leap card.

One shared card only for this talk. Display a single prompt card under the visualiser, on the IWB, or read it aloud. Groups do not hold their own cards yet. Talk for about two minutes, then take three or four contributions. Assign each group its own task card at the start of the next step.

Prompt cards (full text — print one set of four for the whole class)

• Card A — Library sort: Put three story books back on the class library shelf in alphabetical order by the author's surname.
• Card B — PE line: Lead your group from their desks to a quiet straight line at the classroom door, ready for PE.
• Card C — Plant care: Water the classroom plant once, safely, and put everything away.
• Card D — Pack away art: Collect four paint pots, seal them, and return them to the art tray without spills.

Look-fors: pupils naming a detail that changes the result (order of letters, walking without running) versus a detail that does not (who goes first if the line is still straight, the colour of the tray).

Key questions: If we leave this out, does the job still work? If two groups wrote different steps, could both still be right? Yes — more than one clear algorithm can work.

Revoice the keep-or-leave-out idea in plain words: keep what changes the result, leave out what does not.

Before the lesson

Print one set of the four task prompt cards (full texts in the previous step) and assign one card per group. Cut blank step strips or give each group 8–10 small cards and a pencil. Gather the physical props each card needs (see materials list): three class-library books for library-sort groups; plant, watering can and tray for plant-care groups; four empty or sealed paint pots and an art tray for pack-away-art groups. PE-line groups need clear floor space near the door only. Before groups start, pair them for the swap (neighbouring tables: A↔B, C↔D, and so on). If you have an odd number of groups, one trio swaps with a pair.

How to run it (about 18 minutes)

• Class prediction (1 min): Hands up for missing step / wrong order / wrong direction. Note that a useless extra step is clutter, not usually a bug that breaks the job.
• Write (5 min): Groups write 5–8 numbered steps for their prompt. Remind them: only details that change the result.
• Test (3 min): One pupil is the "robot" and may only do what the cards say. Others watch silently, then note one unclear step.
• Break (2 min): Groups change exactly one step (swap two steps, delete one, or reverse a direction). The bug must be deliberate and hidden in the cards.
• Swap and debug (5 min): Exchange card packs with the paired group. The new group runs the broken algorithm, spots the bug, rewrites the broken step, and re-tests.
• Quick share (2 min): Two groups name the bug they found and the fix.

Fold the watchers in: when one group acts at the front, ask the class Where do you think the bug is? and What would you change? — watching is real participation.

Differentiation: support groups with a starter stem on the board: "First… Next… Then… Last…". Stretch groups write a second, shorter algorithm for the same job (stronger keep-only-what-matters thinking).

Common bugs pupils invent: steps in the wrong order, a missing move, "put the book anywhere", "run to the door". Celebrate precise fixes.

Pupils record on paper only. Before they write, show a filled mini-example on the board so the amount of writing is obvious (do not invent journal box names — the page layout is generated later):

• Steps: 1. Pick up the three books. 2. Check each author's surname. 3. Put them on the shelf in surname order. 4. Check the row is A to Z.
• Bug: Step 3 said "put the books anywhere on the shelf".
• Fix: Changed step 3 to "place in alphabetical order by surname".
• Left out: Colour of the covers (does not change the sort).

Circulate and prompt: Write the steps as commands, not a story. Name the bug in one line. What did you leave out, and why was that still fair?

If a group finished debugging early, they sketch a second shorter algorithm for the same task rather than starting a new topic.

Ask only for: the algorithm, the bug, the improvement, and one left-out detail.