Portal 2 is a first person puzzle video game made by Valve. It was released in 2011 for Microsoft Windows, Mac OS X, Playstation 3 and Xbox 360. It is the latest in the long line of games made with Valve's Source Engine, their central proprietary game engine. Since its release, it has won numerous awards for both its design, gameplay, and narrative.  It also went on to host a large amount of user generated content, which expands the game and allows for an extension of play.
The game is set in the now decrepit Aperture Science Facility. The main character, Chell, wakes up as Wheatley (a personality core left over from previous experements) is working to maintain the facility. The two of them set off to recover the portal gun which has been left unattended for years. In the process, they wake up GLaDOS, the villian from the previous installment. She forces you to test (for science), and through your traversal of the Aperture Science Facility, insight is gained into the company's past and the lives of its employees (such as CEO Cave Johnson, Caroline, and the old testers). Eventually, the player defeats the antagonist and escapes the facility, presumably to live happily ever after.
Gameplay in Portal 2 is largely physics-based with an emphasis on puzzle solving. The concept revolves around the ability to place portals, a pair of holes that are linked (that is, whatever goes in one comes out the other and vice versa.) Momentum is maintained through the portal, and new areas can be reached by gaining speed by falling down onto a portal that shoots the player out at high speeds over a gap or other obstacle. During the first half of the game, the player progresses through 19 numbered test chambers, each featuring one or more obstacles. Once overcoming whatever is in the room, the player proceeds through the door that completes each level, proceeding to the next test chamber. The second half of the game takes places in the maintenance part of the facility, the back ends of test chambers, and in storage areas. Since there are no defined starts and ends of levels, that part is much more linear in its construction.
Additionally, the game features an extensive co-op campaign that allows 2 players to progress through a series of test chambers designed for cooperative play. Though the co-op mode shares items and overall design with the single player version, none of the test chambers are repeated. The only character that overlaps is the voice of GLaDOS as she directs you into the bowels of Aperture Science.
There are a variety of puzzle objects in the game such as lasers, laser directing cubes, weighted cubes and spheres (for holding down switches), repulsion gel (blue, for jumping high), propulsion gel (orange, for going fast), automated turrets (which shoot the player on sight), aerial faith plates (which launch the player into the air) and more. Each test consists of some combination of these actively trying to kill you before you reach the ending.
The Perpetual Testing Initiative
Becuase Portal 2 is built in Valve's Source framework, its source is open to the players. This allowed Valve to relase a built-in map and item editor for the game. Players were able to design custom maps that used whatever themes, designs, or items they could come up with. Additionally, Valve's distribution software Steam has a community section where content creators can share, rate, and comment on each others maps. After a time, the ability to create custom co-op maps was also added.  Portal 2 maintains a huge user base in item and level creation, second only to Valve's Team Fortress 2. Custom maps range greatly in difficulty, accomodating both players searching for a higher challenge than that provided by the main campaign and younger players looking for a fun, easy experience.
Portal 2 was very well reviewed across the board. It was praised particularly for its immersive atmosphere, art director, and strong puzzle design/layout. Also mentioned were its adaptability and open environment for user generated content. Additionally, the soundtrack and voice acting of JK Simmons, Stephen Merchant, and Ellen McLain were also highly praised for their performances of Cave Johnson, Wheatley, and GLaDOS respectively. Critics lauded the ease of accessibility of the game, as most players can jump right in. Also, despite having some drama and adult themes, the game is kid-friendly which earned it the eye of multiple parenting sites. 
Reviewers were more critical of the shortness of the single player campaign (it can be completed in 6 hours, on average) and some lack of diversity between Portal 2 and its predecessor. Additionally, the writing for GLaDOS's character seemed to have fallen in quality in their attempt to make her more human and accessable.
It maintains a critic score of 95/100 on metacritic, placing it as the 7th highest rated game of all time.
Because Portal 2 has quite a realistic physics engine and adaptability, players began developing maps that, instead of presenting a puzzle, demonstrated an important physics or computer science concept. Realising this, Valve announced their "Teach With Portals" campaign. They would give free copies of Portal 2 to those teachers who would then design educational levels for use in their classrooms. The campaign targeted subjects such as gravity, momentum, force, optics, and more. 
- ↑ Reily, Jim. "Portal 2 Gets a Final Release-Date", "IGN", 22 December 2010.
- ↑ Molina, Brett. "Portal 2 Skyrim Win Big at GDC Awards" "USA Today", 8 March 2012.
- ↑ Crecente, Brian. "An Insider's Guide to Portal 2" "Kotaku", 5 March 2010.
- ↑ de Matos, Xav. "Co-op Added to Portal 2's Perpetual Testing Initiative" "Joystiq", 16 August 2016.
- ↑ Portal 2 Review http://safevideogames.blogspot.com/2011/04/portal-2-review.html
- ↑ http://www.metacritic.com/game/pc/portal-2
- ↑ http://www.metacritic.com/browse/games/score/metascore/all/pc?view=condensed&sort=desc
- ↑ Ludwig, Sean. Coolest class ever: Valve brings Portal 2 to schools "VentureBeat", 22 June 2012.
- ↑ Monogenis, Harry. Valve brings Portal 2 to schools via 'Teach with Portals' "Destructoid", 20 June 2012.