In 2004, Japanese educator Tetsuya Miyamoto invented a game he thought would help his students hone their math skills. He named it KenKen, a variation on ken, the Japanese word for wisdom. In 2008, Reader's Digest introduced the game to America, and The New York Times began to print versions of the game every day. Other newspapers followed suit, and the game became widely popular.
The goal of KenKen is to fill in a square grid with numbers. The key restriction is that no number can be repeated in a row or column. The squares range from 3 by 3 to 9 by 9, and the numbers used to fill in the grid are the same as the dimensions of the square. For example, a 5 by 5 square uses 1,2,3,4 and 5.
To some, this will sound very similar to another game popularized in Japan called Sudoku. But KenKen adds a twist. Within the square, certain cells are marked off by darker lines. The numbers inside these "cages" must, using an arithmetic function such as addition or division, produce a target number. The function and the target number are specified for each cage.
The game can be simple or hard depending on the size of the square and the design. Requiring a combination of simple arithmetic and complex logic, some large KenKen squares take hours to solve.