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Chamber 13 makes the puzzle more complex by having the solution be a two-step process; you have to press (and keep pressed) two buttons, both of which are ordinarily inaccessible. At the end of the chamber, you gain a little more insight into what GLaDOS has planned for you once you finish all twenty chambers.

[edit] First room

This first room has only a small puzzle, requiring you to fetch a cube to put on a button. Place one end of of a portal on the wall above the walkway, and the other end on a wall accessible to you. Go through the portal and pick up the cube, then fall back down to the button, and leave the cube there. Now that the door's open, you can go back through the portal, through the door and into the main room (but not forgetting to destroy the camera in the room first).

[edit] Main room

Solving the main room requires the use of several different portals, so that a cube can press one of the two buttons on platforms, while you stand on the other holding the door open to fire a portal through into the exit room.

To begin with, the energy pellet needs directing into the receiver so that the platform holding the cube can start moving. Place one end of a portal on the energy pellet's scorch mark on the wall, and the other where the receiver is projecting a red spot onto the ceiling. The energy pellet will go into the receiver, and the platform will start moving.

Next, you need to get the cube off the platform, and onto the first button. Place one end of a portal on the ceiling above the middle of the platform's track, and the other end of the portal on an accessible wall. When the platform's below the exit portal, fall through onto the platform, pick up the cube and walk back down onto the floor with it.

To get the cube onto the button, you need to move the exit end of the portal you just used to above the platform holding the button — aim for the one-dot symbol on the roof. Now, go through the portal with the cube and deposit it on the button. Before you fall back down to floor level, move the exit end of the portal to on top of the two-dot symbol above the second button's platform.

Although you don't have another cube to put on the second button, if you go through the portal again and land on it, the door to the exit room will open, and you can again move the exit end of the portal to an accessible wall in the exit room. Now just fall back to the floor and go through the portal to reach the exit room.

[edit] Exit room

Before you enter the chamberlock, make sure to remove the third camera in the chamber by firing a portal at it. Remember: she's always watching.

As part of a previously-mentioned required test protocol we can no longer lie to you. When the testing is over, you will be "missed".

—GLaDOS