With the promise of cake and grief counseling, you enter Test Chamber 15.

Use Momentum

After you come out of the entry hallway, turn around and look up. You will see a piece of wall sticking out. Make a portal on it, then make a portal of the other color on the floor. Fall in, and as you fall, make a new portal on the floor where you are about to land that is the same color as the one already on the floor. If you do it correctly, your momentum will increase and you will go through the particle field above the glass partition.

Contain the Pellet

Proceed to the area where there is a high energy pellet hitting the wall. This area has two particle fields, one with a pellet-generating machine on the wall, and one with a pellet containment machine on the floor. Put a portal on the dark mark that the pellet has made on the wall, then make another portal that is facing the area with the pellet containment machine. Wait for the pellet to go through the portal, then go through the particle field to the area with the pellet containment machine on the floor. The particle field will make all portals disappear, so the pellet will bounce between the walls above the containment machine, but it will soon disappear after it bounces a few times, so quickly make a portal on the ceiling where the containment machine is shining its orange light, then make another portal on the wall where the pellet has made a dark mark. If you do so before the pellet disappears, it will enter the containment machine. This will make a platform in the wall come down.

Pellet Containment

Stand on the platform that was lowered by the pellet containment machine (there is a line of orange lights on the floor from the containment machine to the platform) and you will reach a new area.

This area is similar to the first part of the Test Chamber. Put a portal on the piece of wall that is sticking out above the hallway, but don't make a portal on the floor here. Instead, go into the side room, where there are some stairs leading up to a pit. Make a portal of the other color on the floor of the pit and fall in, and as you fall, make another portal of that color on the floor where you will land, and keep doing so until you have gone through the particle field.

Make a portal on the piece of wall that sticks out above the two pellet machines, then make a portal of the other color on the floor and fall in, and make more portals of that color on the floor as you fall until you get enough momentum to go through the particle field to the area with the two angled pieces of wall.

You need to make the portal bounce off of the other angled wall instead of the angled wall that it is currently bouncing off of, so make a portal on any of the dark marks created by the pellet, then make a portal that faces the other angled wall (don't put the portal on the angled wall itself). When the pellet bounces off of the angled wall on the right, it will go into the pellet containment machine.

To escape this area with the angled walls, put a portal high up on the ceiling facing the particle field, then fall through portals of the other color on the floor until you get enough momentum to go through the particle field.

There is now an unstationary scaffold moving in a side room above a pit of poison. Don't get on the scaffold when it gets close, because it is just going to go into the wall. Instead, make a portal on the wall at the far end of the poison pit, then make a portal of the other color nearby. Look through the portal, and go through when there is a scaffold beyond it that you can walk onto. From the scaffold, make a portal (of the same color as the one that you just stepped out of) on the far wall and let the scaffold take you back to the door. Look through the portal that you made earlier, and go through when there is a scaffold beneath it. Look to the end of the hallway and you will see the Aperture Science logo on the wall. Make a portal (of the same color as the one that you just stepped out of) on the wall with the logo and let the scaffold take you back to the door. Then go through the portal to the new area.

To solve this area, you need to make the high-energy pellet go into the pellet containment machine that is behind two closed doors. Each door has a dotted line going between it and a red button in a side room near the ceiling. You need to press these two buttons in a short period of time, and then you need to send the pellet through a portal in the floor and a portal of the opposite color on the piece of wall that is facing the enclosed area. To do that, go into one of the rooms with a red button, make a portal nearby, and look across at the other room with a red button, and make a portal of the other color there. Press a button, then go through the portal and press the other button, then quickly make a portal on the piece of wall that faces the two doors that you opened, and make a portal of the other color on the floor where the pellet has made a dark mark. If you do it correctly, the pellet will go through the portals and into the pellet containment machine beyond the two doors that you opened.

Step onto the platform that was lowered when the pellet went into the containment machine, and go into the elevator to go to Test Chamber 16.