Display the ruler image as pupils settle. Take three hands-up answers, not open call-outs. Don't reveal the exact reading yet — note any guesses on the IWB and say 'we'll come back to this at the end and see how close you were.'

Stand at the front with the large demonstration ruler in your hand. Lay each real object against it one at a time so every pupil can see the end-point land on a mark.

Walk the first example slowly and visibly. For the pencil at 12.4 cm, run your finger from 0 up to 12, then count on one, two, three, four millimetres to land at 12.4. Say it both ways: 'twelve point four centimetres OR one hundred and twenty-four millimetres — same length, two ways.' Underline that the digits stay in the same order; only the decimal point slides.

Hold the demonstration ruler up at eye level and lean to one side on purpose. Pupils watch the tip seem to shift by a millimetre. Then come back square-on. That is parallax made visible — the eye must be square over the mark to read true.

On the exercise book (24.7 → 247 mm) just point at the slide. On the pencil case (19.5 cm), pause and ask the class: 'how would I write this in millimetres?' before revealing 195 mm.

This step is pure desk measuring with pupils' own rulers — no IWB widget needed. Pupils each measure three real items at their seats: their own pencil, their own eraser, and their maths copy.

Circulate the room. Catch two slips on the spot:

• Starting at the wrong mark. Many pupils line the ruler's edge against the object, not the 0 mark. Re-show: 'the zero is the start line — line it up at zero, not at one'.
• Reading from the side. Lean across the desk yourself and say 'now look from above — see how the answer changes?'. That is parallax made concrete.

Revoice every reading you hear in both units: 'so 15.3 centimetres is the same as 153 millimetres — the digits don't change, the point moves'.

When you hear a pair disagree by a millimetre, stop the room briefly and ask both pupils to read theirs aloud. Reconciling a disagreement is the moment the precision rule lands. Pupils do not need to write yet — the writing happens in the next step.

This round is the practice bank — pupils measure with their own rulers and the class confirms each reading at the end. Keep the pass-on rhythm brisk so no one is waiting.

• Before pupils start, hold up each of the six objects and name them. Remind them: 'zero at one end, mark at the other'.
• Circulate as pupils measure; the most common slip is starting at the 1 mark or the ruler's edge instead of 0, which makes every reading 1 cm off.
• When five or six pupils have completed the table, stop the room and read the answers aloud together: 'the crayon — eight point six centimetres, eighty-six millimetres'. Pupils self-check; a disagreement is a teaching moment, not a mistake.

Watch for: pupils approximating without lining the end up exactly with a mark — that is the same parallax mistake we warned about in Watch and Notice. 'Eye square over the mark' is the cue.