Part, Part, Whole
A number is made of parts
Before children can add or subtract, they need one quiet idea underneath it all: a number is made of parts, and those parts join back into a whole. Seven is not only a word you say after six. Seven is five and two. It is also four and three, and six and one. The same seven, dressed in different parts. Once a child sees a number this way, addition and subtraction stop being separate tricks to memorise and become two views of the same simple picture.
Part, part, whole
This is what mathematicians call a part–part–whole relationship. Take a collection of up to ten things — counters, blocks, dots on a card — and split it into two groups. Each group is a part. Put the two parts back together and you have the whole again. Nothing was added and nothing was lost. The total stays the same no matter where you draw the line between the parts.
Parts you can see at a glance
The Australian Curriculum pairs this idea with subitising: recognising how many are in a small group at a glance, without counting one by one. This matters because part–part–whole works best when a child can simply see that a part is three, rather than slowly counting it. A ten-frame helps here. Drop six counters into a ten-frame and the four empty spaces appear on their own — six and four, sitting side by side, each small enough to take in with one look.
The parts can move
A powerful next step is realising the parts can be rearranged. The same eight counters can be grouped as three and five, or as six and two, or as four and four. Slide them around and the whole is still eight. Children who grasp this stop believing that “3 + 5” and “5 + 3” are two different facts to learn. They are the same eight, looked at from two sides.
Finding the hidden part
The hardest move — and the most useful — is going backwards. If you know the whole is nine and one part is four, what is the other part? This is the same picture, but now a part is hidden and the child reasons toward it. Reverse thinking like this is exactly what makes subtraction feel natural later on: nine take away four is just “the missing part of nine when four is already there.”
One picture, a web of facts
This is also why part–part–whole is worth far more than drilling addition facts in isolation. A child who has only memorised that “4 + 3 = 7” knows one fact. A child who can see seven coming apart into its parts knows that 7 is 4 and 3, and 3 and 4, and 5 and 2, and 6 and 1, and that nine minus four must be five — a whole web of related facts, all from one picture. Understanding the structure does the work that memorising never finishes.
Parts in the lunchbox
None of this needs to live on a worksheet. A sandwich cut into pieces, some kept and some shared, is a whole splitting into parts. A handful of blocks tipped onto the floor and pushed into two piles is the same idea. The five visualisations below let a child do exactly this on the screen: split a bar, fill a ten-frame, regroup counters, uncover a hidden part, and share out a lunchbox. Each one shows the same truth from a different angle — the parts can change, but the whole holds.