The 3DS is able to render 3D without the need of 3D glasses. Neat trick, but the 3DS has another ace up its sleeve.
The 3DS can, as pointed out on blog Tiny Cartridge, continue to play music while in sleep mode when it is placed on a DS Lite. When placed on a DSi, however, the 3DS goes into standard sleep. Oddly, the DSi goes into forced sleep mode when near a DS Lite, so the 3DS trick works in reverse!
This isn't just the music, though. The game is still "playing", because the 3DS doesn't read that the lid is shut. The reason for this is due a magnet in the lid that determines whether the 3DS is closed or not. Ah crap, now I've told you how the trick works, ruining the illusion. Sorry!