Hold the metre stick up against the door as pupils settle so the comparison is real, not imagined. Give five seconds of quiet think-time, then take three hands-up answers, not open call-outs.

You are listening for the idea that a door needs a big unit and a pencil case needs a small one. Don't name metres and centimetres yet — that lands in the next step.

Walk each example aloud, one at a time, on the IWB.

• Ruler (30 cm) — point out it is short, so centimetres make the number sensible.
• 1 m length — pause here. Say 'one metre is one hundred centimetres' and have the class echo it. This is the key fact of the lesson.
• Desk (60 cm) — ask whether metres or centimetres feel right for a desk before you reveal.
• Bookshelf (1 m 20 cm) — the first one big enough that metres become handy; link 1 m 20 cm to 120 cm.

Aim to cover about four objects in this slot: one or two small ones the class measures at their desks, and one or two longer ones a pair measures at the front. For each object, have the class vote cm or m before anyone measures, then measure and check the vote — that vote is your pacing lever and keeps the watching pupils thinking.

As pupils measure at their desks, circulate and catch alignment slips on the spot. Revoice a pupil's reasoning aloud ('I chose centimetres because a copybook is small') so the choosing is visible to all. Keep each front-of-room watch beat short.

• Reading slip to watch: lining the ruler or metre-stick start exactly with one end before reading the far end.
• If you have only one metre stick for the room, keep it at the front for the longer objects and let pupils share rulers for the small ones; the centimetre readings can be renamed to metres at the end if you wish.

Split the slot clearly: pupils measure the desk and one small object at their seats with their own rulers; a pair measures the classroom door and the corridor at the front while the class watches and checks. The class confirms each reading aloud.

Take the lengths in size order so the unit choice gets harder: the desk (cm), the small object (cm), the door (m), the corridor (m). The corridor is measured once by the pair, not by every pupil — they lay the metre stick end over end and the class counts the metres together. For the stretch, the win is spotting that, say, a desk at 60 cm and a chair back at 60 cm are the same length, or that the door at 2 m is the same as 200 cm.

• Watch for pupils who write 200 cm for the door — both are right, so use it to revoice '2 metres is the same as 200 centimetres'.
• If time runs short, keep the desk and small object as seat work and demonstrate just the door at the front, leaving the corridor for the next session.

Keep this brisk. Recap the cm-for-small, m-for-big rule and the 1 m = 100 cm fact one last time, then point forward to renaming so today's choosing-the-unit work feels like the natural lead-in.