Balconia Math : 21 Marbles

Try a new kind of math game in line with the Math Revolution envisioned by innovators like Youcubed and AOPS. Designed especially for children 6-11, "21 Marbles" doesn't quiz or time the player, instead it let's them enjoy thinking and reasoning.
A puzzle game's level progression is a natural "low floor, high ceiling" exercise. In the case of 21 Marbles, while the puzzles can become challenging enough to be enjoyed even by adults, the game has a friendly progression of difficulty that keeps everyone on board.
For the youngest players, it has the additional benefit of practicing the most basic arithmetic facts in an instinctive way. For the adults, it's like a relaxing peg solitaire game.
There are no instructions because the rules are natural and intuitive. 21 marbles - of colors and designs for all tastes - are won one by one. In every level, the recovered marbles - each representing the number 10 - break randomly into pairs of smaller marbles: either into a 9 and a 1, or an 8 and a 2, or a 7 and a 3, or a 6 and a 4, or two 5s. The marbles get positioned randomly on a board, and can be merged back together into 10s, by knocking together pairs that sum to 10. It is always possible to recover all 10s, but some thinking ahead is required. The "undo" button is always there to let you trace back your steps and find your mistakes. Learning to think ahead is not optional, and is the most important educational aspect of this game.