Children discover all the ways to partition a number (e.g., 5 = 2+3 = 4+1, etc.). Understanding number bonds develops flexibility in thinking about whole numbers and their parts.
Use five-frames or objects. Show 5 objects split into different groups (2 and 3, 4 and 1, etc.). Draw or record the combinations with drawings.
You already know how to look at a small group of things and know how many there are without counting one by one — that's subitizing. And you've practiced combining small groups to find a total. Number bonds bring those two ideas together and add something new: the idea that any number can be *broken apart* into pieces, and that the pieces always add back up to the whole.
Think about 5 red apples in a bowl. You can put 3 on the left and 2 on the right. Or 4 on the left and 1 on the right. Or even all 5 on one side and none on the other. No matter how you split them, you always have 5 total. A number bond is a picture of this relationship — a whole number at the top connected to two part numbers below. It shows the part-part-whole relationship: the two parts fit together to make the whole.
The key discovery for each number is that there are only a few ways to break it apart. For 5, the bonds are: 0 and 5, 1 and 4, 2 and 3, 3 and 2, 4 and 1, and 5 and 0. Finding all of them teaches you to think in a complete, organized way — not just one split, but all the splits. This organized thinking is the beginning of addition and subtraction facts, which you will use soon.
One more important idea: the order of the parts doesn't change the whole. 2 and 3 make 5. 3 and 2 also make 5. This is the same group of apples, just rearranged. Noticing this — that swapping the two parts gives the same whole — is your first peek at a big mathematical rule that shows up over and over as you learn more math.