Nintendo has released a new 3D game for the DSi in Japan which uses the devices in-built cameras to track the position of your eyes in relation to the two screens, and then delivers a 3D game world in line with the position of your head.
It sounds a little bit confusing at first, but if you watch the YouTube demo of Rittai Kakushi e Attakoreda, you quickly see that the depth illusion works. Really well.
Inside the looking glass
So it looks like you are actually able to find the shapes required in the game's puzzle's inside or somewhere behind the DSi's 2D screen.
How does it do this? Simply by using the front camera of the DSi to track your eyes' position in relation to the angle the two screens.
The game then changes the perspective of the 3D camera in sync with your eyes, to create this fantastic 3D illusion.