After you make it through the live-fire course, you reach Test Chamber 17.

Weighted Companion Cube

In this level you are introduced to the weighted companion cube, a weighted storage cube with a heart on each side. You need to take the cube all the way to the end of this Test Chamber with you.

After you receive the Weighted Companion Cube, take it to the end of the hallway where there are some ledges that you can't reach. Place the cube on the floor and jump onto it to use it as a step up. Be sure to take the cube with you after you reach the top of the wall.

Before you proceed, make sure you are carrying the cube. You may notice a dark mark on the wall. To stay safe, you need to hold the cube in front of you to block the pellet. This might make the pellet start to bounce around at random, so you might want to wait for it to disappear. When you reach the stairs at the end, quickly walk sideways and backward into the hallway on the right, and keep walking backward to block the pellet from the second machine. After you walk backward down some stairs, you will be safe from the pellet.

You will reach an area where there is a pellet containment machine on an angled wall. Shoot a portal onto the other angled wall (the one that doesn't have a pellet containment machine on it), turn around and look for the dark mark on the wall where the pellet is hitting it. Make a portal of the other color there. Wait for a portal to enter the containment machine. A slow platform will rise.

Before you drop down to the lower level, go back to the narrow hallway. The nearest pellet machine is no longer shooting pellets, but you will still need a pellet, so go down the narrow hallway and create a portal on the far wall where the other pellet machine is still shooting pellets. Make sure to remember which color of portal the pellets are being shot at. Then take your companion cube and fall down to the lower area.

In this area, there is a piece of wall sticking out, with a hand print on the wall nearby, and a small room behind the piece of wall where there are more writings on the walls. Across from the piece of wall that is sticking out, there is a room with two red buttons, one near the entrance and one near the back wall. The red buttons will open two doors that are blocking the path to a pellet containment machine. To make a pellet go into it, you will need to use the cube as a step to reach the red button near the back wall, then put the cube onto that red button, then make a portal on the piece of wall that sticks out, and quickly stand near the nearest red button, but off to the side where the pellet won't hit you. After the pellet passes by, quickly step on the red button to open the door. The pellet will go through the nearest door, and the door that the cube is holding open, and will enter the pellet containment machine. This raises another platform.

Now take the cube into the other side room, where a pellet shooter and a pellet containment machine. The walls here do not take portals, so use the companion cube to make the pellet bounce into the containment machine.

With all three platforms raised, make a portal above the ledge, and take the companion cube with you as you jump across the raised platforms and reach the raised hallway at the end. When you reach the red button, you are informed that you are required to incinerate the companion cube. To proceed, place the cube on the red button, go through the door that opens, then push the small red button beyond the door to open the shutter on the incinerator. You have a limited amount of time to go get the companion cube and drop it into the incinerator. After you do so, the exit will open. Go into the elevator to go to Test Chamber 18.