Traffic lights, a cake recipe, a bus-stop announcement: each one follows a fixed order of steps. What happens if one step is missing or in the wrong place?

Today you write precise step-by-step instructions for a real classroom job. Then you change one step so the job goes wrong, swap with another group, and hunt for the mistake they hid in their steps. Finding that mistake and repairing it is the main work of the lesson. After that we try the same thinking on a robot route on the board.

Watch how I write steps for watering the classroom plant. Which details would you keep, and which would you leave out?

Here is the classroom job we are all thinking about for the next two minutes: put three story books back on the class library shelf in alphabetical order by the author's surname.

With your group, call out steps you would keep in the instructions and details you would leave out. Be ready to say why. Which step, if it were missing, would make the job fail? Which detail sounds important but does not change where the books end up?

Quick class vote first: which change is most likely to make a job fail — a missing step, steps in the wrong order, or a wrong direction? (An extra step that does nothing is usually clutter, not a bug.)

Your group has one task card. Work through these jobs in order:

1. Write a clear algorithm on the step cards (5 to 8 steps).
2. Test: one person follows the steps exactly while the others watch.
3. Break exactly one step so the job fails.
4. Swap card packs with the group you are paired with.
5. Find and fix their bug.
6. Test again to check the fix works.

On your Investigation Journal page, record four short notes about the work you have just done. Keep each note to one or two lines. Here is what a finished set looks like, using the library job as the example:

• 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: The colour of the covers, because it does not change the sort.

Now write your own four notes: your group's steps, the bug you found in the other group's cards, the fix that worked, and one detail you decided to leave out because it would not change the result.