The game includes three difficulty levels; easy, medium and hard. You will be able to select the difficulty level when you click the key "New game”.

For your convenience, when you select the tile, you will have the particular line, column and box highlighted.

If you want to retract, click the 'undo' button. If you have changed your decision, you can restore your movements by clicking the 'repeat' button.

If you get stuck or have no idea about the missing number, select the square and click the 'solve' button. The missing number will appear in the selected square. However, bear in mind that each prompt results in a 1 minute penalty added to your time.

When you complete the whole board, check its correctness by clicking the 'check' button.

If any numbers are wrong, or any squares remain empty, correct them immediately. For your convenience, the incorrectly filled squares will be highlighted.

Your time is measured by the game clock which is positioned at the top of the screen. Your final score, received after each correct completion of the board, depends on the puzzle difficulty level and the amount of time spent on the game. For your best results, bookmark ''Scores”. You can also compare there your own results with the top scores, achieved by other users.

**How to play Sudoku**

Beginners are advised to start with easy Sudoku puzzles. Begin with the columns, lines or boxes containing the smallest number of empty squares. You will often find the areas, where the number of empty squares varies from 4 to even 1. One way to figure out which number can go in each space, is to use the process of elimination.

If the process of elimination does not work in particular lines, columns or boxes, do not randomly pick numbers for empty squares. Instead, you had better move to another section of the board and complete the empty squares by using the process of elimination. As a result of this, you can get a clue about the number you failed to figure out before.

To solve the easy Sudoku puzzles, it seems enough to use the process of elimination. However, if you jump to higher levels of difficulty, the correct completion of the board will require more of your logical reasoning.

**History of the game of Sudoku**

The first version of Sudoku, called ''Number place”, was published in May 1979, by the American entertainment magazine ''Dell Pencil Puzzles & Word Games”. The puzzle was invented by Howard Garns, an American architect, when he was 74 years old. In 1984, the editors of Nikoli Publishing in Japan decided to include the Number Place in the magazine ''Monthly Nikolist" and called it ''Suuji wa dokushin ni kagiru" ("the digits must remain single"). The name was too long so it was subsequently abbreviated to Sudoku, with the word 'Su' meaning ''number" and 'Doku' meaning '' single". However, it was not until 2005 that the game gained enormous popularity because the magazines all over the world began to publish it on a regular basis. The game of Sudoku soon replaced most of traditional crosswords and its popularity resulted in the publication of numerous books about how to solve the puzzles. The world went crazy about the game.

**Some interesting facts about Sudoku**

In 2005, two mathematicians, Bertram Felgenhauer from Dresden University of Technology and Frazer Jarvis from the University of Sheffield ,calculated and proved the existence of 6 670 903 752 021 072 936 960 valid sudoku solution grids. However, the number of essentially different solutions, when symmetries such as rotation, reflection, permutation and relabelling were taken into account , was shown to be just 5 472 730 538.

The minimum known number of givens, leading to a unique solution, is 17 digits in the whole diagram. Not enough initial numbers is more likely to lead to a multiple possible sulution situation. However, it does not mean that each diagram containing 17 givens leads to a unique solution.

Some players are also interested in generating the Sudoku grid. The Big Bang Play website uses an algorithm to create new puzzles. It can generate 100 000 grids within a few seconds.