For five months in 2008, Valve Software was making a version of Portal 2 that wasn't going to include Portals. And it was going to be a prequel, set in the '50s. The game, based on an internal prototype called F-Stop, was built around a new gameplay mechanic that thrilled people at Valve. » 4/21/11 4:30pm 4/21/11 4:30pm