Display the 500 ml bottle and the 1 litre carton side by side as pupils settle. Give five seconds of quiet think-time, then take three hands-up answers, not open call-outs. Write 1 l = 1,000 ml on the board once someone reaches it, but don't over-explain yet — that is the next step's job.

Board the worked example before touching the displays. Write the two steps as you go: 1 l → 1,000 ml, then 1,000 + 500 = 1,500 ml. Say why the number grows: ml is the smaller unit, so it takes more of them.

Slip to head off: writing 1 l 500 ml as 150 ml or 1,500 l. Stress the litre alone is already 1,000 ml.

Then step through the three converter displays, asking for a prediction each time:

1 l to ml → 1,000 ml (× 1,000).

2 l to ml → pause for prediction, then 2,000 ml.

1,500 ml to l → number gets smaller; 1.5 l, read as 1 l 500 ml. Head off 15 l.

For 1 l 500 ml to ml, stay on the board with the worked example (the converter takes one value in one unit); same two board steps. This is the method they use next.

This round is for talking it through together — pupils take turns at the board and the class agrees or corrects out loud.

For each call, a different pupil sets the From and To units on the converter and reads the result. Ask the class each time: are we multiplying by 1,000 or dividing by 1,000? For the mixed prompt, combine 1 l 250 ml into 1,250 ml by hand first (1 litre = 1,000 ml, add 250 ml), then enter 1,250 ml on the converter — the tool takes one value in one unit, so the combining is a board move before the tool. Watch for the pupil who converts 1 l 250 ml as 125 ml — remind them the 1 litre alone is already 1,000 ml. Keep the four values in the order named so the round builds from the clean × 1,000 up to the mixed amounts.

These are the practice questions — pupils take turns at the board, check each answer, and the class confirms before moving on. Keep the board work brisk rather than over-explaining.

The practice set rises in difficulty: the first two are clean single-step conversions, the last two need the litre part combined or split. For the third, combine 1 l 250 ml into 1,250 ml by hand first, then convert 1,250 ml back to litres on the converter to check the figure. For the final challenge the converter shows 2.75 l; make the mixed read explicit on the board — 2 whole litres, and 0.75 of a litre is 750 ml, so 2,750 ml = 2 l 750 ml. Ask the class to predict each answer before the pupil presses Check. The point is that final split — that is where the whole round has been heading.