Take three hands-up estimates, not open call-outs. Don't reveal the actual length yet — the lesson is about estimating first, then checking. More or less than ten? is enough to get every pupil committing to a guess.

Walk each example aloud, one at a time. Each time, point to where the object starts: see how it sits on the zero, not on the 1.

• Crayon (6 cm) — point to the zero end first, then trace to the 6.
• Pencil (9 cm) — ask the class to read the far mark before you say it.
• Copybook edge (11 cm) — emphasise reading past the 10: ten, then one more.
• Rubber (4 cm) — a short one, to show the same rule works for small objects.

This round is for talking it through together — a pupil reads at the board and the rest of the class agrees or corrects out loud.

Call up an individual pupil to read the object currently on the board to the nearest centimetre. Before they read, ask the class: is the zero lined up? If a pupil reads from the 1, that is the teaching moment — the measurement comes out one centimetre short. After each pupil, change the object on the board to a new length (use the widget controls) so the next pupil reads a fresh length. Rotate four pupils across four resets.

This round is for pupils to measure at their desks with their own rulers — circulate and catch alignment slips on the spot. The class reads aloud and you reconcile any disagreement as you circulate.

The make-or-break error is starting from the 1 instead of the 0; that gives a reading one centimetre short every time. Watch for it as you walk. Keep a small shared bank of 'about 10 cm' objects on the front desk so no pupil stalls choosing one. Fast finishers find which two of their objects differ by the most centimetres, and by how many.

Keep this brisk. The natural next step is the centimetres-and-millimetres lesson — flag that the ruler has smaller marks the class will use next.