edit : meh that's too bad.They're pretty much relying on Nature and your point of vision. Since you eye's collect 2 images instead of one, the 3DS uses a screen that produces 2 images, and those 2 images will combine to create the 3D effect. But it only works when you're perfectly alinged with the device.
I just hope its a pretty large window.
or I'll just need to train my eyes to go crossed...
...
CROSSFIRE.
sorry.