You know how to put two groups together. Now try the opposite: split one whole into two parts. The two parts always add up to the whole.
Mia has 6 apples. She makes two piles: 2 apples and 4 apples. Together, they still make 6.
There is more than one way to split a number. 5 can be 1 and 4, or 2 and 3. Check by adding the parts back together.
A number splits into two parts in more than one way. The parts always add back to the whole.
Look at the stars above. What two groups make 6?