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Portal, a puzzle-oriented add-on game for Half-Life 2: The Orange Box and Half-Life 2: The Black Box takes players' perceptions of spatial reality and literally throws it for a loop. Using a portal gun, gamers place portal ends on any surface and then run through them. The laws of physics will be bent to the breaking point, because gravitational pull can turn into upward or horizontal force to travel across distances. As puzzles become more challenging, some with dangers like gun turrets, players will be forced to think quickly, but must be careful not to fall into an infinite loop and get stuck.