Look around the room. Where can you already see a perfectly square corner?

Hands up and point to one you can see from your seat.

First we make the tester together. Take one sheet of paper. Fold it carefully in half and press the crease flat. Fold it in half again the other way and press that crease flat too. The sharp folded point is a perfect square corner. That point is your right-angle tester.

Now watch the tester held into four different corners. Notice which ones it fits exactly, with both edges lying flat, and which one it does not fit. If both edges lie flat, the corner is a right angle. If a gap opens, it is not.

Think back to the paper tester. Would both edges lie flat against this corner, or would a gap open? We will look at the corner of a square card, the corner of a rectangle, a slanted corner of a triangle, and a wide open corner.

Now we sort corners together in four rounds that get trickier as we go. A pupil comes to the board for each round while the rest of the class agrees or disagrees out loud. Round 1 has two clear corners, Round 2 adds a rectangle, Round 3 mixes four corners, and Round 4 tests the tricky ones, including a corner that is almost square but not quite.