Because we use base ten, knowing the combinations of two numbers that make ten is really useful. Children can fill in the blanks as we say this poem (and may even learn the poem) before the concept of addition has been introduced.

There are many ways we can make ten
If we think fast and think fast again
Who is as fast as bright sunshine
And ready to bring me one and ____
Who is next now don’t be late
I am wanting two and _____
Clouds above and stars in heaven
Who remembers three and _____
Easy as breaking ice with sticks
Someone give me four and ___
Heavens above! Sakes alive!
It’s like a mirror! Five and ____
We’re not tired; we can do more!
We can give you six and ____
It’s as easy as climbing trees
When we give you seven and _____
If you know you can join in too
When we give you eight and ___
Now our task is almost done
But first we’ll give you nine and ___
Tomorrow we’ll do this all again
But today we’ll end with zero and ___.