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Mental state in terms of challenge level and skill level, according to Mihaly Csikszentmihalyi.[needs simplifying]

Worry is an emotion. It means that one feels stressful or he or she is panicking about something. It is felt as anxiety or concern about a real or imagined issue. Most people experience short periods of worry in their lives, but it is not normally a problem. In fact, a small amount of worrying may even have good effects, if it makes people take precautions or be more careful.