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.

Ruler, 30 cm: short enough to hold, so centimetres make a sensible number.

Metre length, 100 cm: pause here. One metre is one hundred centimetres, the key fact; have the class echo it.

Desk, 60 cm: ask metres or centimetres, still centimetres.

Bookshelf, 1 m 20 cm: first one big enough for metres; link 1 m 20 cm to 120 cm.

Close: small things in centimetres, longer things in metres, and a metre is always 100 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.

Open with one full estimate-then-measure model on a desk at the front before any pupil work.

• Show the desk width. Class votes: cm or m.
• Class estimate, e.g. about 50 cm. Board it with the unit.
• Measure carefully (align the zero). Board the real length, e.g. 60 cm.
• Compare: estimate was a little low, unit choice was right. That is the habit for every length today.

Split the slot: pupils then do their own desk and one small object at seats (estimate and unit first, then measure); a pair does the door and corridor at the front while the class watches. Before each front measure, take a quick class estimate and unit vote, then measure and check.

Take lengths in size order: desk (cm), small object (cm), door (m), corridor (m). Corridor once only, stick end over end, class counts metres aloud.

After the measures, board the rename and keep it visible:

• Desk reading: 60 cm.
• Key fact: 1 m = 100 cm.
• 60 cm is 60 out of 100 cm, so write 0.6 m.
• Chair back 0.6 m matches desk 60 cm.

If a pupil offers the door as 200 cm, revoice both forms: 2 m is the same as 200 cm.

• Slip to watch: measuring before writing an estimate; treating 0.6 m as 6 cm; writing the decimal with no link to hundredths of a metre.
• If time runs short, keep the modelled desk plus pupil desk and small object, demonstrate just the door at the front, leave the corridor for next session; still board the 60 cm to 0.6 m rename.

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.