Here, I've just used portals to get energy from the red, three-pointed power emitter to the power socket just to my left, and then portalled over to a platform which travels along the beam of white energy. Note the sinister observation room to the right. The glass is only distorted from the outside, but that's something you find out later in the game.
And then it gets better. Just when you're nodding your head and deciding that some puzzlers can be fun, the whole gameplay shifts up a gear, the plot (yes, it's a puzzle game with a plot) blossoms like a hydra. Then you reach the final puzzle, which makes everything that came before it look like Tetris, and then the game ends, at which point something ten times more awesome than everything so far happens.
And you're left staring at the empty computer screen with a lump in your throat, adrenaline washing round your brain, emotionally and intellectually wrung out. Valve have tricked you. They lured you in by saying this is a little puzzle game with an interesting gameplay mechanic, and then they suckerpunch you with the best gaming experience you have ever had. And it's all over in five hours.
Seemingly without trying, this pint-sized, funny-looking, not-quite-an-FPS, not-just-a-puzzler game has strolled into the all time top ten and taken a seat in the VIP area alongside Deus Ex, Half-Life, and System Shock 2.
The cake, incidentally, is not a lie. My companion cube told me so, before I killed him. I'll miss that little guy.