Here is a photo of a pencil lying along a ruler. The tip is sitting somewhere between the 16 and 17 centimetre marks. How long is this pencil, as exactly as you can read it? Hands up: what number would you write down?

Watch closely at the front. The ruler has long lines for the centimetres and tiny lines in between — ten millimetres in every centimetre. We will measure a few real objects against it, so watch where each one ends and where the eye needs to be to read it true.

Now it is your turn at your own desk. Take your own ruler and measure three things: your pencil, your eraser, and your maths copy. For each one, read it to the nearest millimetre.

Say each length aloud two ways: in centimetres with a decimal (like 13.8 cm) and in whole millimetres (like 138 mm). Remember to look straight down, eye square over the mark, before you read. Each pupil's pencil, eraser and copy are different lengths, so readings will not match.

Before the stations, watch one worked reading together. An object ends between two tiny millimetre marks, past the 8.6 line but not quite at 8.7. It sits closer to 8.6, so the nearest millimetre is 8.6 cm (or 86 mm). When an end falls between two marks, choose the nearer one, not a half.

Six real objects are set out as stations at the front: a crayon, an eraser, a glue stick, a highlighter, a marker, and a colour pencil. In your maths copy, rule three columns and head them Object, cm, mm. When it is your turn at a station, measure that object with your own ruler to the nearest millimetre.

What we learned today

• The smallest mark on a school ruler is one millimetre, and ten of them make one centimetre.
• To read accurately, line the 0 mark against the end and look straight down, eye square over the mark.
• The same length can be written two ways: centimetres with a decimal, or whole millimetres.

Coming up