Sudoku
From Tardis Wiki, the free Doctor Who reference
A sudoku was a number-based puzzle which was solved with pen and paper. Sudokus of various difficulties were printed in the local newspaper of Winchelham. (PROSE: I Am A Dalek)
Within the sphere chamber, Rajesh Singh reported to Yvonne Hartman that he was on "Sudoku book 509". (TV: Army of Ghosts)
At the end of a long day, Colin Colchester-Price relaxed by doing Sudoku. (AUDIO: A Kill to a View)
