There is a cube in a glass area, and a water tube and a blue gel tube. First, put a portal on the ceiling above the glass area that contains the cube. Then put a portal of the other color under the spot where the blue gel is falling. This will cover the cube in blue gel, making it bounce so much that it cracks the glass.
The cube will continue to bounce around indefinitely. Catch it, then take it over to the water tube and put it under the water. The blue gel will wash off so the cube won't bounce anymore. Place the cube on the red button. This makes an elevator below the exit door come down.
Have a portal under the blue gel, then make a portal below the elevator. Move that portal before the gel falls. This will coat the floor with blue gel, allowing you to bounce up to the elevator. But in order to make the elevator go up again, you have to get the cube off of the red button.
No problem. Just put a portal on the ceiling above the cube to make the blue gel fall onto it. The cube will bounce off of the red button, making the elevator go up. Follow the path to the elevator.