Searching for Sudoku on yields over 5,000 results. Since 2004, when Sudoku became a global phenomenon, hundreds of newspapers have begun publishing a daily puzzle. But unlike the venerable crossword puzzle, there are a finite number of possible Sudokus. Are we at risk of running out?

According to research by Bertram Felgenhauer and Frazer Jarvis, Sudoku addicts will be able to get their fix for a long, long time. Using a computer program, the two found there are about 6.7 x 1021 (6,700,000,000,000,000,000,000 or 6.7 sextillion) possible games of Sudoku. To put this in perspective, the approximate age of the Earth is 4.5 billion years; beings we would consider anatomically human have been around less than 200,000. People could play one billion unique games of Sudoku per day, and there would still be enough puzzles to last until the sun explodes in 5.6 billion years. So Sudoku fans need not worry about getting bored—with that many puzzles, there is no need for repeats.