Peter Silk of Nukezilla pulled out his pocket microscope and took a gander at the Nintendo 3DS screen to help everyone better understand how the parallax barrier works. See, in order to deliver two different images to each of our eyes, a parallax barrier shields certain pixel columns from each eye, which is how stereoscopic 3D works. These two separate images combine to give the illusion of depth. It's like the old red and blue 3D, only instead of filtering colours, they're actively controlling what images your eyes are seeing.
On the left of the image we see the screen with the 3D slider turned all the way down. This is 2D, so the entire screen is lit.
On the right side, the 3D slider is turned up, and alternating columns of pixels are dimmed out. It's a simple solution that produces a very impressive effect.
So next time someone asks you how the 3DS works, just send them the link below. If they are still confused, pat them on the head and make reassuring sounds to keep them from being driven mad by this dark magic.
The 3DS Under a Microscope [Nukezilla - Thanks John!]