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This chamber is more an exercise in ignoring extreme pessimism from GLaDOS, rather than a logical challenge. The level begins with GLaDOS saying:

The Enrichment Centre regrets to inform you that this next test is impossible. Make no attempt to solve it.

—GLaDOS

Despite the pessimism, this chamber is easily solvable. To begin with, create a portal at ground level so that you can take a cube up to the platform, using the fixed orange end of the portal there. Once a cube is up there, walk off the platform and through the door into the other room. Create another portal somewhere in the room, walk through it, collect the cube and put it on the button to open the door and allow you to reach the chamberlock. You cannot take a cube directly through the door, as there is a vapourisation field there similar to the one in front of chamberlocks, which will vapourise any cube you try to take through it.

Fantastic. You remained resolute and resourceful in an atmosphere of extreme pessimism.

—GLaDOS