To introduce conflict, writers of fantasy fiction often place limits on the magical abilities of wizards to prevent them from solving problems too easily.[1]:616 In Larry Niven's The Magic Goes Away, once an area's mana is exhausted, no one can use magic.[5]:942 A common limit invented by Jack Vance in his The Dying Earth series, and later popularized in role-playing games is that a wizard can only cast a specific number of spells in a day.[5]:385