Part 21

Shoot a portal at the white wall, then another at your feet. Turn around and shoot a portal at the concrete slab above the door.

Go to the ledge, shoot a portal, and jump down into it. Enter the small room.

Press the button, then crouch under the moving piston to the left and hug the wall.

Shoot one portal where an energy ball hits a wall, and another at the laser of a turret. Repeat for all four turrets to destroy them with the energy ball.

Look towards the door to see it on the other side of the piston.

Shoot one portal at the concrete block in the back left corner. Shoot another portal where energy balls hit the walls. As soon as the ball goes through, shoot a portal at the far left wall (near the button on a pedestal) and another at your feet. While falling, press 'use' to activate the button just as the energy ball passes through.

Look to see the scaffold is now running. Run to it and get on when it is near.

Once it reaches the other side, look down into the small room.

Inside, shoot a portal at either the left or right walls. Press the button. Shoot the other portal on the wall that emerges on the other side of the room. Hurry through the portal.

Once you land, go right to the ledge. Jump down quickly and shoot a portal in mid-air where you are going to land to fly over to the box.

Grab the box and wait for the scaffold to return. Jump on the box and scaffold, and deposit the box on the other side. Wait for the scaffold to go back, then jump down to the lower room.

Shoot a portal on either side, press the button again, and shoot a portal on the emerging wall. Run through. Grab the box and go back under the piston. Deposit the box on the button.

Shoot a portal over the gap at the wall near the door. Look left and down to a platform you were on earlier. Jump down and shoot a portal at your feet.

On the other side, run through the door and down the hallway. Turn and face the water on the wall to the right.

Shoot a portal at the lowest piston. Go to the pit left of the door and shoot a portal near the wall nearest you. Jump into the portal.

While flying, aim at the next tallest piston and shoot the portal you did not just come out of (the one in the pit). Continue alternating portals on the pistons to ascend.

On the last piston, look above the door for a slanted piece. Shoot a portal there to be sent flying to a piece of land.

Enter the chamberlock. This leads to Test Chamber 19 (TC19).

Run to the panel. Head around the corner.

Once inside the room, look slightly left to see a commentary node floating above a scaffold.

Turn and face the two slanted pieces of ground. Shoot a portal at the right piece. Wait for the energy ball to bounce off the left piece before shooting the other portal at it. The scaffold will start moving. Jump on.

While riding, duck under the piston to avoid being pushed off and killed.