In Portal 2 the player returns as Chell, having been in stasis for several hundred years while GLaDOS and the rest of the Aperture Science facility has fallen into disrepair. Chell is awakened by one of GLaDOS's personality cores, and inadvertently wakes GLaDOS; GLaDOS is immediately displeased at Chell's return and begins testing her again through numerous chambers as she rebuilds the dilapidated facility. Portal 2 continues to challenge the player through numerous platforming and physics-based puzzles using the portal gun, a device that can create a temporary hole between any two flat surfaces. Other gameplay elements are added to Portal 2 including tractor beams, laser redirection, and special paint-like gels, taken from the Independent Games Festival-winning DigiPen student project Tag: The Power of Paint, that imparts special properties to objects it touches such as increased momentum. Portal 2 also includes a two-player co-operative mode, each player-character a robot that has become self-aware from GLaDOS' influence, and requires the players to work together with their own portal guns to complete each level.