Here are five everyday things: a lunchbox, a full water bottle, the classroom door, a bag of crisps, and the teacher's desk.

Which one do you think would be the easiest to estimate well without measuring? Which one would be the trickiest?

Watch the three anchors at the front, one at a time: a one-metre stick, a one-kilogram bag of sugar, and a one-litre bottle of water. These are our anchors.

For each object that follows, notice how a good estimate compares it to one of these anchors first (about half the sugar bag, a bit longer than the metre stick) before anything is measured.

The ruler below shows the first anchor in use: an object edge a little past the metre stick, read to the nearest centimetre.

Today we estimate then measure three things in the room, anchoring each one first: the teacher's desk width (anchor: the metre stick), a full lunchbox (anchor: the sugar bag), and a water bottle poured into the jug (anchor: the litre bottle).

For each one we agree a class estimate out loud, write it down, and before we measure, everyone predicts whether we will be over or under. Then we measure and read the actual.

Now we practise reading four rulers to the nearest mark, each one a little finer than the last. Read each length in turn: the crayon, the pencil, the strip, and the long edge. On the fine rulers, count the small marks between the whole centimetres — each small mark is one tenth of a centimetre. Before each reveal, predict which mark the reading will be nearest.