Trace your finger right around the edge of a book so the class sees the round trip start and finish at the same corner. Take two or three hands-up answers, not open call-outs. You are listening for the idea of adding up the edges — you don't need the word perimeter yet, it lands in the next step.

Work each shape live — touch the first side, say its length, then add the next as you go. Hold out for pupils to keep the running total with you.

• Triangle — three sides, so three numbers to add. Ask a pupil where the trip finishes.
• Square — all four sides match, so four fours; a good spot to link to the 4-times table.
• Rectangle — the long side twice and the short side twice; ask which sides are equal before you add.
• L-shape — count the sides first. The step is where pupils skip or double-count, so touch each side deliberately.

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

The perimeter readout is hidden while pupils work, so nobody can read the answer off the screen — they must add the sides themselves. Have the pupil at the board trace each side with a finger as the class counts along. The confirmed totals are: triangle 12 cm, square 20 cm, rectangle 16 cm, L-shape 16 cm. The L-shape is the one that catches people — a skipped or double-counted side gives the wrong total, so slow right down and touch each of the six sides once.

This is the practice round — real objects circulate around the room (or sit at stations) and pupils measure each one with their string and ruler. Keep the pass-on rhythm brisk; the class confirms each reading aloud at the end.

The common slip is the string sagging or cutting across a corner instead of following the edge — circulate and catch that on the spot. For the final compare, listen for pupils reasoning from the side lengths (this card's long side is bigger, so its way around should be longer) before they measure.

Close by pointing back to the L-shape as the trickiest case — six sides to add, none skipped, none doubled. That care with counting sides carries straight into the rectangle work next lesson.