It was no coincidence that Bornstein came across Numbrix in Parade magazine. The puzzle was developed by Marilyn vos Savant, who writes the magazine's "Ask Marilyn" column and it first appeared in her column in 2008; today, Numbrix also is available on the magazine's Web site and as an app for iPhones and iPads [source: Parade]. Regardless of whether players find the puzzle online or in print, the rules are the same and quite straightforward.
At first glance, Numbrix looks a bit like a crossword puzzle: The playing space is a nine-by-nine grid with mostly empty spaces that need to be filled in. But that is where the similarities with crossword puzzles end. There are no cleverly worded clues, but rather a handful of numbers between one and 81 already in place in the grid.
Very simply, it's the task of players to fill in all the blanks so that there is a continuous chain of numbers between one and 81 [source: vos Savant]. The only rule is that numbers inserted must be along the vertical and horizontal axis, not diagonal. Another way to think of it is that you've won the game if you are able to take a pen and trace a vertical and horizontal path from one to 81 without ever lifting your pen off the paper. The online version of the game has a "hint" button that fills in a number to provide a clue, but otherwise memory and logic are your only tools.
Keep reading to learn more about the big brain behind Numbrix.