Portal looks sick-inducing at times. How does your thought process work coming up with puzzles. Ever have to rethink? To watch it's very confusing...

Kim Swift: The way we design our puzzles is that we all sit down together as a team - the artists, programmers, level designers. We all sit down and pretty much decide goals for each level; we want the player to use these particular gameplay objects say a box to open a door, or to use a 'fling' - which is when you put one portal on the wall and one on the floor and you use your momentum from standing high up on a ledge to gain acceleration to toss yourself out of the other portal. So we say 'Okay we want to use one of those' and we figure out the layout of a level - we draw it out on the whiteboard and then one of us goes and designs it. Then we playtest it and see if it works or not.

Everyone's been very impressed. Were other teams in the company eager to nick your ideas? For Episode Two?

Kim Swift: I don't really know if that's the case! We definitely got a lot of help from people trying to fit our technology into the Source engine!