Portal/Chamber 18

Chamber 18 is the hardest test chamber yet in Portal. I hope you have your fingers (and your in-screen portal gun) ready! Chamber 18 is cold, a bit inhuman and a bit scary, but it's quite fun. Chamber 18 is the hardest chamber you may face, but look on the bright side: you might get cake after you complete the next chamber.

Part 1
Lethal water to take note of here! To get past the lethal gap, put a portal across the gap and put another one near you. Past the gap, there is an area that has stairs, a radio and other stuff you may see in the game. Now to get past the first three floors, put a portal on the ceiling where you may land on the floor and put another on the wall .Make the portal on the wall a bit lower than your level, since you can't just walk there, you have to jump there.

Part 2
Go to the next hall and press the button to open a wall (a section at least). Some turrets are guarding this next room. Now just put a portal on the moved wall then to somewhere to get enough speed to fling to the next room. This next room is usually a cube room and a weighted storage cube. Bring it and quickly get back by placing the portal on a platform which is really on the lethal water. Now quickly get there before you take too much damage. Release the cube on the button to take you to the next part.

Part 3
This part is all about:
 * Jumping
 * Portals
 * A single fling

Now on your left, you see the floor on the bottom. Put a portal there and another on the platform you see straight ahead. Now once you jump, you may see the floor of the higher platform. Shoot a portal there on the platform floor and then wait for yourself to go through the portal. Don't place the same colored portal you placed on the lower platform or things can get tricky. The last platform is slanted, meaning you need to fling to the exit. Put the portal there then you are flying. Proceed to the elevator as GlaDoS talks to you.