This is a thoughtful activity for those those odd moments (maybe when you need a cup of coffee) and want to keep the children engaged and thinking.
You need a pencil and paper
To start, on a napkin or scrap of paper write the numbers
1, 2, 3, 4, 5, 6, 7, 8, 9
Now ask your child to choose three numbers that will add to 12.
Once done, see if they can find a different combination of three numbers that add to 12...
and another...
and another...
How many different ways can they find?
(there are 7 different ways, did they find them all?)

Talk about the number combinations.
Do they spot any patterns in the numbers?
What happens if you include a zero as well as the digits 1 to 9? How many different ways are possible now?
You can take this further by asking them to pick four numbers with a total of 12. How many different ways can they find?