Even grown-ups who can add 7 + 5 in their head instantly once counted one finger at a time. The bridge between finger-counting and mental math is the idea of "number partners of 10."
A number partner is a number that, when added to another number, makes 10. 1's partner is 9. 2's partner is 8. 5's partner is 5 (itself)!
Once you can recall these partners instantly, without even thinking, addition and subtraction get much easier later on.
Why 10? Because the number system we use is built on groups of ten. 11 means "one ten and one." 23 means "two tens and three."
So when adding two numbers, it's very useful to first make a 10, then add what's left over. Let's try 7 + 5. 7 only needs 3 more to become 10.
So we borrow 3 from the 5 to give to the 7, making 10, and add the leftover 2: 10 + 2 = 12. This is called the "make ten" strategy. Knowing your number partners well is what lets you use this strategy freely.
A tool often used for this is the "ten-frame" — a grid of 10 boxes that you fill in with dots or circles.
Since you can see at a glance how many boxes are filled and how many are empty, you instantly feel how many more you need to make 10. Showing filled and empty boxes is much easier for kids than just asking "what's 10 minus 7?" with numbers alone.
A sense of number partners comes in handy everywhere in daily life — figuring out change at a store, or calculating time (like "what time is 10 minutes from now?").
It also helps later when learning to add two- and three-digit numbers. "Carrying" is really just the same idea: fill up to 10, then carry the rest to the next place. Building a strong sense of the number partners of 10 makes every step after this much easier.
When practicing with your child, it helps to ask about different partners over and over. Try a quick back-and-forth game: "What's 6's partner?" "What's 3's partner?"
Eventually your child will be able to answer instantly, without counting on fingers. Another good practice: line up 10 counters, blocks, or candies, cover or remove a few, and ask "how many are missing?"
On our activity page, you can look at how many dots are filled in a ten-frame and first guess how many more are needed to make 10. Then watch an animation fill in the answer. Practice it again and again to build a strong feel for number partners.