Not sure how 3D on the 3DS works? Let Hoagie from Day of the Tentacle explain.
While some of you may already know the technical background behind the 3DS' glasses-free technology, this video is still worth watching, because it gives you a few great examples of "cross-eye" 3D (including game footage), instances where you can cross your eyes while viewing two images to form a third, 3D picture.
A warning, though: too much cross-eye hurts, which is the price you pay for getting that 3DS effect without the 3DS.
How the 3DS Works [Kombo]