Lemon

As Chel makes her way up through the old test chambers the rooms get newer. By the 70's Apature has gone from an elite testing ground with the very best of subjects to a company on the verge of bankruptcy having to pay people off the street 60$ to take the tests. Then in the 80's they couldn't get any volunteers and had to start using company staff. The head of the company’s health is failing due to the toxic nature of some of the stuff they are testing, and he mentions a plan to copy his mind into an AI, and failing that the mind of his second in command.

The later levels introduce 2 more liquids, an orange one that shoots things forwards and a white one that makes any flat surface able to hold a portal. I knew from the ads a while back about the blue and orange, but was surprised by the white when I came across it.

In the 80's levels Chel finds Potato GlaDos and pokes her onto the spike at the end of the portal gun to carry her around. They then make their way up to the modern levels where Wheatly (the orb) is trying to run his own experiments with turrets welded onto boxes as subjects. GlaDos tries the old Star Trek ploy to take down an AI with a logical paradox, but Wheatly is too stupid to understand it and the plan fails. Now They are stuck having to run through the old experiments that GlaDos came up with that Wheatly found until an opportunity presents itself.