Certain planes feature aggressive, multi-segmented creatures that perish when all their segments are eaten by the player's creature; however, they can eat segments of the player's creature to regrow their own. These creatures release many cells upon death, which can restore the health of the player's creature, temporarily increase the size of its mouth, or cause it to sprout decorative protrusions. Players are not required to eat these or any other organisms; they may travel to higher or lower planes at any time. Being defeated by aggressive creatures does not result in death, but causes the player's creature to float to a higher plane. In the Flash version, the player can replay the game with a jellyfish-like organism by defeating an aggressive creature on the bottom plane. If the player reaches the bottom again, the creature there is their original worm-like creature, and defeating it starts the game over as that organism.
The game was released in March 2006, after two months of development—during which Chen and Clark taught themselves Flash programming. The game's source code was later released in 2009. The game's score was composed by Austin Wintory. A PlayStation 3 version was announced for the PlayStation Store in May of that year, and was released in February 2007. Chen had graduated by that point, and had founded Thatgamecompany, which handled the conversion to the PlayStation 3; Nicholas Clark was one of the employees and served as the game version's designer. Impressed by Flow, Sony provided Thatgamecompany with finances, supplies, and additional staff, and offered them a three-game contract; the PlayStation 3 version of Flow was the first of these. Chen originally believed the conversion could be completed in four months and that it would be ready for the November 2006 launch of the PlayStation Network. However, when it was finally released in February 2007, it did not include "half of the original design".
Reviewers were mixed in their opinions on the gameplay; Roper said that "simply see[ing] what's next" was enjoyable, but acknowledged that the game's activities were limited beyond this aspect. Leadbetter went further, saying Flow "feels like a tech demo in many ways", and that its small amount of gameplay existed primarily "to act as a tour guide" through the visuals and audio. Freeman said that "it would not be unfair to describe Flow as empty and without substance." Navarro concluded that Flow was enjoyable for the few hours needed to complete it, but it lacked "a lasting gameplay experience"; he attributed this to the game's "heavy emphasis on aesthetics over gameplay". Luke Mitchell of PALGN, however, felt that "anything more complicated would take away from the friendly nature of an experience of this type."
Reviewers found the game to be largely unchallenging. Roper stated that "there really isn't any sort of a challenge here", particularly due to the player character's inability to die; Leadbetter summed up the game's difficulty as "negligible". However, when reviewing the PlayStation Portable version of the game, Justin Calvart of GameSpot took the view that "the game's difficulty masterfully scales to match your skill level", so it does not become overly "frantic or frustrating". Reviewers were universally dismissive of the multiplayer mode, with Calvart saying that "whatever it is, it's not good". Leadbetter called the PlayStation 3 version's multiplayer "little more than an afterthought", while Navarro said it "doesn't add a lick of depth to the experience".
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