You already know these maths blocks. The flat, the rod, the small cube — you've used them since 2nd Class to build numbers like 234 or 1,028. Today they get a new job.

From now on, the flat stands for one whole, the rod stands for one tenth (0.1), and the small cube stands for one hundredth (0.01). The blocks haven't changed size on the desk. What's changed is what each one is worth.

In a moment, four decimals will appear on the place-value mat. Watch carefully where each block lands, and especially what happens when a column is empty. That empty column is doing important work — and we'll find out exactly what.

Watch four decimals being built on the place-value mat. Each one uses the U / t / h columns — units, tenths, hundredths. Notice where the zero sits in each number, and what it tells us.

First, 0.3 — three rods in the tenths column, no small cubes. Then 0.27 — two rods and seven small cubes. Then 1.05 — one flat in the units column, no rods, five small cubes. Finally 2.30 — two flats and three rods, with no hundredths at all.

Time to build some decimals on the U / t / h mat. The teacher will call out a decimal. First, think to yourself which columns the blocks need to go in. Then one pupil will come up to drag the blocks into place on the board.

Watch carefully as each block goes down. At the end of the build, give a thumbs up if you agree, or a flat hand if you'd change something.

Listen carefully when a decimal has a zero somewhere — those are the ones that catch us out.

Five decimals to build, one after another, getting harder. Pupils take turns at the IWB. Before you tap Check, say aloud which column each block went into. The activity will show ✓ if the build is right, or 'not yet' if a block is in the wrong column.

Watch the board carefully. The zero-traps are coming: 0.06 has no tenths, 1.07 has no tenths, and the last one means exactly the same as 2.5.