Warnings Active: Orb Hitting Body, Orb Receiver, Dangerous Water, Jumping Into Portal, Flying Out Of Portal, Don't Drink The Water

Now the particle walls you see at the end of each level - and rarely mid-level - are a major obstacle to your progress. The key manuever here is making a portal while in flight. I will explain: Make one portal on the floor and another on the pushed-out wall above you. Jump into the hole. The momentum will not carry you over the first time, which is why, while falling out of the portal, you must move the "floor" portal into your parth so that you land in it. Thus you go into the floor, out of the wall, back into the floor, out of the wall (with more momentum) and over the field.

Now, in the next section, there are two particle fields. You must get the orb into the receiver. Remember that walking through the field removes all established portals, so you must send the orb flying toward the area before you cross through. First, make a portal on the wall where the orb is impacting. Next, make another on the same wall to send it through the particle field into the receiver area. After the orb passes through the first portal and out of the second, walk through the field. Make a portal on the wall that the orb will hit and another above the receiver. Don't worry about the orb disappearing; since you've walked through the field, the orb will simply hit blank wall and bounce back. Walk onto the now-active lift.

In the third room, you must use the midair portal generation technique used earlier. Make one portal on the wall behind where you came in, then go into the side room and make a portal in the pit there. It is again important to remember which color you've used for which. The momentum gained by falling into the pit will launch you onto the portal-able white tile. While in midair, move the "floor" portal in front of you so that you fall into it, repeating the momentum-gaining trick established earlier. Now you should be over the wall and presented with the following: an orb emitter, a receiver, a jutting-out section of wall, and another field that contains two angled pieces of wall. Do the recursive portal maneuever again (no pit is required because the whole room is portal-able) and you should find yourself on the other side. Make a portal on the right wall, facing the angled surface on the right. Wait for the orb to come and bounce once (so that it is headed for the flat wall) and then put a portal on the left angled surface. Because of the redirection of energy, the orb should go straight out of one portal, hit the right angled surface, and then bounce towards the emitter. Do another recursive jump by putting one portal on the ceiling and another on the floor. No pushed-out surface is required.

In the next part of this testchamber, the platforms are constantly coming towards you. Make one portal at the end of the hallway and the other on the wall to your right. Jump through it when there is a platform to land on at the other side. Do the same for the next part, except keep the portal that you just exited and move the portal that you walked into. Now you can see into the room; launch one portal into the room (at any point) and put the other on a wall that you can walk into.

This final part of Testchamber 15 does not use the established momentum method. Rather, take either of the lifts to a room with a button. When you are up, make one portal on the floor where the orb is bouncing and the other on the pushed-out section of wall. Then immediately push the button. One of the two doors will open. Making sure that the orb is between the doors and the pushed-out section of wall (so that you can trap it in this path until both doors are open) shoot a portal at the other control room with a button (you should use the portal that was on the wall and not the floor, so the orb doesn't escape while you are preoccupied). Then make a portal in your own control room. Cross over and push that button as well. If timed properly, the orb should be in the receiver. Now take the lift and walk into the ending elevator for this testchamber.

Portal Walkthrough