A game is a structured activity, usually undertaken for enjoyment and sometimes also used as an educational tool. Games are generally distinct from work, which is usually carried out for remuneration, and from art, which is more concerned with the expression of ideas. However, the distinction is not clear-cut, and many games are also considered to be work (such as professional players of spectator sports/games) or art (such as jigsaw puzzles or games involving an artistic layout such as Mah-jongg solitaire). Key components of games are goals, rules, challenge, and interactivity. Games generally involve mental or physical stimulation, and often both. Many games help develop practical skills, serve as a form of exercise, or otherwise perform an educational, simulational or psychological role. The requirement for interactivity should put games such as jigsaw puzzles and mah-jongg solitaire into the category of solo-player puzzle rather than being a game.