Vitrified doors

To gain the Door Prize achievement, you're going to have to use the intercoms on the five Vitrified doors located around the original Aperture Science labs. They are pretty obvious when found, but they are hidden in some less-than-obvious places. The first two can be found fairly easily, right after you finish the long elevator ride after the first set of Repulsion Gel test chambers. The final three are in the Control Room, which can't be missed. Follow the video to find the hidden door for the corridor containing the final three doors. Remember to hit the intercom on all five doors; otherwise you will not get the achievement.