Look at the picture on the board: an empty box, with a pile of little centimetre cubes beside it.

Roughly how many of those little cubes do you think would fill the box right up to the top? Have a good look before you guess.

Three boxes come up on the board, one after another, each already filled with little centimetre cubes: a box 5 cm long, 3 cm wide and 2 cm high; a box 6 cm long, 4 cm wide and 3 cm high; and last a cube with every side 4 cm. Watch how the number of cubes grows each time.

For each box, count one flat layer of cubes first. Then count how many layers are stacked up. One layer, times the number of layers, gives the whole box.

The last box is a plain cube: every side is the same length. Together, tell me what happens to the count when all three sides are the same length.

We build and measure at the board together. Whoever is at the board drags the sliders while the whole class works each answer out loud.

First set the box to 3 × 2 × 2. Then grow it to 5 × 4 × 2. Then raise the height to 3 cm.

Each time, work out the base layer first, then how many layers high the box is, and call out your prediction before we read the volume off the tool.