International Sudoku Day is celebrated on Sept. 9 to appreciate one of the most popular puzzle games.

The logic of Sudoku is to complete a 9×9 grid by filling each row, column, and 3×3 subgrid using the numbers from 1 to 9, with the numbers only occurring once.

Here are some interesting facts about the addictive game:

1. The name Sudoku comes from the Japanese word "suuji wa dokushin ni kagiru," which means, "the numbers (or digits) must remain single."
2. The current version of Sudoku is believed to have originated from the puzzle game "Number Place" published by Dell Puzzle Magazines in the 1970s.
3. The first Sudoku World Championships were hosted in Italy in 2006.
4. Sudoku is rated from one star to five stars; five stars being the most difficult one. The world's hardest Sudoku developed by Arto Inkala, a Mathematician from Finland in 2012, is described as grade 11, according to The Telegraph newspaper.
5. There are more than 1 billion valid Sudoku puzzles.
6. Although it appears to be a mathematical game, sudoku is a logical game that can be played with numbers, symbols, letters, or pictures and does not require mathematical skills at all.

Here are some interesting quotes about the game that you may like to read. (Courtesy: quotes sayings, More famous quotes)

1. "I can't travel without Sudoku."- Robert Ballard
2. "Who knew we had all this O.C.D. in the world? Well actually, I suppose it's pretty obvious. It explains Sudoku, doesn't it?"- Bruce Vilanch
3. "The most common anxiety I hear about learning to program is that people think it requires a lot of math. Actually, most programming doesn't require math beyond basic arithmetic. In fact, being good at programming isn't that different from being good at solving Sudoku puzzles."- Albert Sweigart
4. "There seem to be two main types of people in the world, crosswords and sudokus."- Rebecca McKinsey
5. "Crossword puzzles, Sudoku ... I'm good at all those things. It's not daily, but I'll do stuff on the airplane. I love playing chess. It's my favorite game."- Larry Fitzgerald
6. "I read every country's perspective on an issue. I also play many games like Bridge, Scrabble and Sudoku online."- Indra Nooyi
7. "My secret vice is Sudoku puzzles. Can't stop playing them. My parents are accountants. I blame them entirely."- Lisa Gardner