Cube (Portal 2) - Valve Developer Community

Valve's Secret Weapon

Valve On Portal 2: Spoiler Interview Part One

Portal 2 - Valve Developer Community

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Portal 2 Puzzle Maker: Cubes - Valve Developer Community

Game portal, companion Cube, Chell, aperture Laboratories, GLaDOS

Source - Valve Developer Community

Portal - Tutorial - Box Dropper - Valve Developer Community

Source - Valve Developer Community