Dream world (also called dream realm or illusory realm) is a commonly used plot device in fictional works, most notably in science fiction and fantasy fiction. The use of a dream world creates a situation whereby a character (or group of characters) is placed in a marvellous and unpredictable environment and must overcome several personal problems to leave it. The dream world also commonly serves to teach some moral or religious lessons to the character experiencing it – a lesson that the other characters will be unaware of, but one that will influence decisions made regarding them. When the character is reintroduced into the real world (usually when they wake up), the question arises as to what exactly constitutes reality due to the vivid recollection and experiences of the dream world.