Agar.io

io is to grow a cell on a Petri dish by swallowing both randomly generated cells, known as "agar", which slightly increases a cell's mass, and other smaller cells (players), without being swallowed by even larger cells. The browser version currently holds five game modes: FFA (Free-For-All), Battle Royale, Teams, Experimental, and Party. The mobile version of the game includes Classic (like FFA), Battle Royale, Rush Mode Burst, and Relax (a game mode in which all of the other players are video game bots that progressively increase in size). The goal of the game is to obtain the largest cell; players must restart from a small cell when all their cells are eaten by larger players or fountain viruses.

Players can change their cell's appearance with predefined words, phrases, symbols, or skins. The more mass a cell has, the more slowly it will move. Cells will gradually lose a small amount of mass over time.

io was created by nineteen-year-old Brazilian developer Matheus Valadares, written in JavaScript and C++. io on 28 April 2015. It is named after agar, a substance used to grow cell cultures. Valadares continued to experiment with adding new features—experimental mode was created specifically for this purpose.

He thought about adding more complex biological features like photosynthesis and mitochondria, but ultimately decided to strip the game down to its current simple mechanics. In-game advertisements covered server costs.

Agar.io was praised for the addictive nature of its simple gameplay and graphics. PC Gamer included it on its list of the best browser games.

Engadget described the game as "a good abstraction of the fierce survival-of-the-fittest competition that you sometimes see on the microscopic level." TouchArcade praised its simplicity, strategic element, and "personality". It was chosen by SFGATE as App of the Week in August 2015. Criticism was mainly targeted towards its repetitiveness and the controls of the mobile version.