Portal begins by demonstrating through hands-on experimentation how portals work, and which surfaces you can use. When you first gain the Aperture Science Handheld Portal Device, or the "portal gun", you can only place one portal, with the other fixed. It doesn't take long to grasp. "When we introduce a new mechanic, we'll playtest that and see if the player got it right away, if he was really confused, or just didn't get it at all, we have to go back to the drawing board," says Swift. "We just kept iterating on that until we found a good way to train players. Now we're able to teach them really complex mechanics and tricks." The benefits of this approach are apparent throughout. By the time the second portal is introduced, you're well into your stride, and happy to start firing them off. Left-click for blue, right-click for orange. Another example is the "funnelling" toggle in the options menu. Enabled, the game gives you a bit of help jumping or bouncing things through portals, without stopping you dead because you snagged the boundary by a pixel or two. Good work, playtesters.