Here are three instruments, and on each one the pointer has stopped between two marks: a ruler measuring a pencil, a jug of water, and a weighing scale with a bag on it. What is each one reading? And how did you decide about the marks that have no number written on them?

The interactive shows a pencil measured against a ruler. Watch where the pencil's right edge lands and how we work out what one small mark is worth before reading it off. Then we will read a real scale and a real jug the same way — always check what one mark is worth before you compare.

Now you measure with your own instruments. First, on your own at your desk, estimate then measure a pencil or copybook edge on your ruler to the nearest millimetre. Read it square-on. Say what one small mark is worth.

Next, take turns on the shared group instruments. Estimate then weigh a small object on the group scales to the nearest 50 g. Estimate then pour water into the group jug and read the level to the nearest 100 ml, with your eye level with the mark. Each time, estimate first, and work out what one small mark is worth before you give your answer.