Portal/Chamber 17

The weighted companion cube
This is your introduction to the weighted companion cube. He will accompany you for the entire chamber, providing any assistance you require. When he drops from the transport tube, pick him up, and use him to climb the staircase.

You should now see a corridor with energy pellets bouncing back and forth - hold the cube in front of you to deflect the pellets harmlessly. After going through both corridor sides, you should reach the main test area where you need to raise three platforms, by bring pellets to the three receivers.

Platform 1
One of the platforms is easily opened without assistance from the cube. Place a portal within one of the two corridors, and the other on the ceiling above the angled wall. The pellet will bound and hit the receiver.

Platform 2
The doorway to the side contains a pellet launcher firing a pellet perpindicular to the receiver, but there is no method to place a portal. However, you can bounce the pellet using the cube into the receiver.

Platform 3
As with the first platform, use a pellet from the corridor, and place the other portal on the extruding wall.

To get the pellet into the receiver, you need to place the cube onto the raised button, and walk onto the other button as soon as the pellet passes by.

Exit
Once all three platforms are raised, shoot a portal onto the ceiling, and bring the cube with you. Jump across all three portals and onto the exit platform, and place the cube on the next button.

GlaDOS will explain that the cube has grown too intelligent and must be euthanized. Proceed into the open room and open the incinerator, and drop your companion cube into it. The chamberlock will then open.