Over the past couple of decades, sudoku has gone from a unique Japanese import to a worldwide craze and finally to a daily puzzle ritual for millions, if not billions of people.

If you are one of them, we offer a wide range of challenging Sudoku puzzles in an easy-to-use ebook format, but something that is perhaps equally puzzling is why Sudoku has become so universal.

The game of nine grids of nine squares each is played by children and adults alike, giving it a level of all-around popularity matched by very few other games. Only crosswords and wordsearches are perhaps as widely played, and they have limits to their design that Sudoku does not.

The key to a truly accessible game is that it needs almost no additional context outside of the game itself, making it a pure test of skill or intellect. 

Crosswords rely on a broad lexicon; if you do not know the definition of a given word, you have little chance of solving the puzzle, even if you are given additional letters through solving other words.

Even Picross, a similar game of mathematical reasoning where you draw pictures on a grid, requires you to understand what the grid blocks are trying to represent.

Meanwhile, Sudoku only requires you to know the simple rules of fitting all nine numbers in each grid without duplicating a number horizontally or vertically. You do not need any advanced skills in arithmetic, nor do you need to apply knowledge outside of the puzzle itself.

This focus on cognition and deduction means that as soon as you are old enough to understand the basics of Sudoku, you are capable of solving it.

This has caused it to be very popular amongst children and become a benchmark puzzle used in neurological research; as the game is universally understood, it can be used as a benchmark for testing certain types of cognition without potentially corrupting the result.