Questions / instructions:
Re-check the adjustment of the balance before starting. Throughout this activity encourage the student to explain what they are doing and thinking.
Place the 4 boxes in a row in front of the student, in the order A, B, C, D.

1. Here are four boxes of Pebbles. They look the same, but they each have a different weight or mass. Think about how you could put them in order from the lightest to the heaviest — then tell me how you would do it using the balance. Don't use the balance yet.

If the student simply says "Weigh them"...
How would they go about weighing them?

Put the placement mat in front of the student.
2. I want you to use this balance to help you work out the order of the objects, from the lightest to the heaviest. Tell me how you are working it out as you are doing it and put the boxes in order on the placement mat.

Once the student has arranged the boxes in order from lightest to heaviest, record their decisions on the recording sheet.
3. If you had to explain to someone else in your class how to work out the order from lightest to heaviest, what would you tell them to do?

Commentary:
Students were better at doing than explaining. Explanations improved a little after students did the task.