I.

Introduction

Puzzle, problem designed as a mental challenge. Solving a puzzle often provides a rewarding experience, helping the solver to think in a new way. Puzzles may be distinguished from games, a broad class of competitive activities primarily directed at amusement. Some games, such as roulette and other games of chance, may require little or no ingenuity. A puzzle, however, either is constructed intentionally or is used to perplex and to stimulate thinking of potential solutions. Nevertheless, some classic games—such as solitaire, go, chess, and checkers—include numerous puzzles.

The broad appeal of many types of puzzles is demonstrated by crossword puzzles, which appear daily in nearly every newspaper around the world; jigsaw puzzles, which are enjoyed by youngsters and the elderly alike; and mechanical puzzles, such as the Rubik's Cube, which sold 200 million units in the early 1980s.