Sudoku
These puzzles were created to be done using pencil and paper, and there are many books that contain collections of these puzzles with tips and strategies for completing them. They also appear in newspapers and magazines: typically, a different puzzle appears every day and the solution is printed somewhere else or in the next issue. Later, small handheld machines were created that generate Sudoku puzzles for the user to solve. More recently, though, the game can be played on the Internet or on mobile apps.