For this, you’ll need to ensure the children have rulers that are split into cm and mm (Even inches if you want to make it a bit more complex).
- Give the children a starting number (e.g. 13cm). Tell them to put their finger on it.
- Tell them to add or subtract numbers – start slowly at first, but get faster…
To make it more complex for older children, you could use the mm as decimals or even as fractions (for example, add 1 and 1/2 – 1cm, 5mm).