Hold up the corner of a book and say the words aloud first: this straight corner is a square corner. Then open a real pair of scissors slowly to three positions: a narrow opening, a square-corner opening, then a wide opening. Take two or three hands-up answers only — don't settle the argument yet, the tester in the next step is how the class decides.

Fold an A4 sheet in half, then in half again, to make a sharp right-angle corner tester. Model four holds, working the discovery live — do not narrate the answers before the class predicts:

• A narrow opening — the tester's arm sticks out past it. Ask: smaller or bigger?
• A square corner — the tester fits exactly. This is the right angle.
• A wide opening — the tester slips inside with a gap. Ask: smaller or bigger?
• A tiny, barely-open pair of strips — clearly smaller.

Three buckets only: smaller, exactly, bigger. No degrees, no protractor. Keep the pupil words 'smaller / bigger than a square corner'.

The IWB drag-sort is the canonical delivery here. If you have printed angle cards and a paper tester, you may run the same sort with physical cards on the board as an optional upgrade — but the interactive alone is enough; you do not need the cards.

Talk this one through together — pupils take turns at the board and the class agrees or corrects out loud.

Before a pupil drops each card, ask the class to predict with the paper tester in mind. Save the near-right-angle card for last — that is the one where the tester really earns its keep. If a pupil drops the near-square card wrongly, don't correct it yourself; hold the paper tester to the board card and let the class see the fit.

This is the practice round — pupils take turns at the board, check each answer, and the class confirms before moving on. Keep the board work brisk rather than over-explaining.

On the near-right-angle round, hold the paper tester up before the class calls it. On the final mixed round, let a pupil sort all four, then the class checks each drop in turn. Callout to weave in: when an angle is nearly a square corner, use the paper tester before you decide.