Take three hands-up answers, not open call-outs. Some pupils will already know the words right angle or square corner — accept both. Give five seconds of quiet think-time before hands go up.

Make a folded checker in front of the class first — fold once, fold again the other way, hold it up, then test it against a known right angle like your book corner.

• Page corner — the checker fits exactly. This is a right angle.
• Window corner — also fits. Right angle.
• Flag point — the checker is too big for the pinched point; the corner is smaller than a right angle.
• Open book corner — the checker is too small; the corner is bigger than a right angle.

Ask each time: does the square corner fit exactly? Hold out for a prediction before you confirm.

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

Hold out for the reasoning, not just the sort: ask would the paper checker fit that corner exactly? before each drop. The party-hat point is too small; the open-fan corner is too big. Revoice a good answer: so a right angle is neither pinched nor wide open — it is exactly a square corner.

Every pupil folds a checker and hunts at their own desk — this is independent practice, not a turn-by-turn board loop. Circulate and prompt: does it fit exactly? Watch for pupils calling a slightly-off corner a right angle because it looks square — send them back to the checker. In the last minute, take two or three pupils holding up a find for the class to confirm. For the stretch, a rectangle has four right angles; a strong pupil should check all four, not assume from one.