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You start in a small box called the "relaxation vault", unable to get out. After a few seconds, GLaDOS will introduce you to the game:

Hello and again welcome to the Aperture Science Computer-Aided Enrichment Centre. We hope your brief detention in the relaxation vault has been a pleasant one. Your specimen has been processed and we are now ready to begin the test proper. Before we start, however, keep in mind that although fun and learning are the primary goals of the enrichment centre activities, serious injuries may occur. For your own safety, and the safety of others, please refrain from…

—GLaDOS

At this point, the voice is subsumed by static, and nothing can be made out, until GLaDOS counts down to the vault opening. Such outbursts of static will happen frequently when you are about to be given potentially useful information, just to make things harder.

The vault opens with a portal; a hole in space from one place to another — in this case, from inside the portal to a wall facing it. Looking through this portal, you can see yourself in the vault. You'd better get used to such paradoxes, as Portal is full of them, and indeed, they are the basis for the game.

Exit the vault, and proceed to the next room, which contains the first puzzle. This puzzle is designed to get you acquainted with the basics of the game, and all that is required is for you to pick up the block and put it on the button to open the door. With the default keybindings, pick up the block by pointing at it and pressing E, then move it to the button and press E again to release it. The door will open, and you can proceed to the chamberlock to get to the next level.