Potato

Well the game took a very unexpected turn.

After running through several more experiments, the orb show up again with a plan to take down GlaDos. So they escape into the innards of the facility and first sabotage all the turrets then cut off the supply of deadly neural-toxin. They then manage to swap GlaDos' AI out for the orb, connecting her AI to a potato battery, but just as Chel is about to escape the orb goes mad with power and holds her back. Potato GlaDos then remembers that the orb was designed as a way to restrict her intelligence, by programing it to be the stupidest AI possible, and now he's running everything.

While throwing a tantrum for being called stupid, the orb tosses Potato GlaDOs and Chel down to the bottom of the facility. A bird flies off with the potato GlaDos and Chel is left to make her way up the very first test chambers of Aperture Science, guided by the prerecorded voice of the company's first president. The layout of this area is huge geodesic sphere with 50-60's style tech, and the experiments so far revolve around manipulating this blue liquid that makes things super bouncy.

The change to this retro lab and the turn of the orb is as big a twist and surprise as when GlaDos tried to kill Chel in the first game and then she had to make her way through the back areas.