If you’ve ever tried to finish a really hard Sudoku, you know it can be quite a challenge. The Windows 8.1 Sudoku game starts by asking whether you want an Easy, Normal, or Hard game.

Sudoku has a fascinating history. Apparently, the first puzzles of this type to appear in print were published in Dell Magazines, starting in 1979. They were known as Number Place. It appears as if they were created by a 74-year-old retired architect named Howard Garns from Connersville, Indiana.

These puzzles first appeared in Japan in the Monthly Nikolist paper, starting in April 1984. The name Sudoku is an abbreviation of a lengthy Japanese name that means, roughly, “the digits are limited to one occurrence.”

The object of Sudoku is to arrange the digits from 1 to 9 in a 9 x 9–grid, so that each column and each row has one of each digit. To make things a bit more complex, each of the nine 3 x 3–blocks must also have all the digits from 1 to 9.

You’re presented with a grid, pre-filled with some numbers. Your challenge is to come up with an arrangement of digits that fits all the rules.

The game play in this app is a bit confusing. First, tap or click an empty square. A box appears around the square. Then, tap or click one of the digits. To clear a square, tap or click it, then tap or click the blank “number” below the 9.

If you put a number on the grid and the number already exists in the same row or column, you goofed, and the number appears in red. If there are no immediate conflicts, the number appears in white.

The Internet has about 200 billion free Sudoku grids.