Look at this ruler. A pencil is lying on it, and the tip is sitting somewhere between the 16 cm mark and the 17 cm mark.

Watch as we hold a large demonstration ruler up at the front, lay a pencil along it, and read where the tip lands. The pencil reaches exactly 12.4 cm — that is twelve whole centimetres plus four more millimetres.

Now the trick. To write the same length in millimetres, slide the decimal point one place to the right: 12.4 → 124 mm. Same length, two ways.

Two more examples to lock it in. We lay an exercise book against the demonstration ruler — it reads 24.7 cm. Slide the decimal: 247 mm. Then a pencil case: 19.5 cm. What is that in millimetres? 195 mm — same length, two ways.

Now you try. Take out your own ruler and measure three things at your desk to the nearest millimetre: your own pencil, your own eraser, and your maths copy. Read each measurement aloud as you take it. When you and the person beside you have measured the same item, compare your answers and sort out any disagreement.