FUN is a resource in Cargo! The Quest For Gravity. It is an abstract concept that serves as both currency and energy. FUN is created through enjoyment by the Buddies, who crave FUN and will abide by any means to experience it. FUN is then used to power machinery, and create Color, which is then turned into Goodness.
FUN is used by the player to obtain pieces of the Earth from the Stratosphere. By creating FUN with the Buddies, the player can open an in game menu that shows the available objects These objects can vary from houses and pieces of Earth that provide more maximum Buddies, or landmarks that must be returned to Earth to progress the story. FUN is also used as a currency to buy vehicle parts that the player has not collected for themselves.
FUN is the concept that entertains the Buddies, which is a primary goal of the Demiurges - by replacing humans with Buddies, they will be able to remove all "undesirable" aspects of human workers and always have Buddy subordinates.
For a list of achievements relating to FUN, check out Achievements.