A Rumble Pak is an add-on for the Nintendo DS that provides force feedback to immerse the player into the game. It's about the size of a Game Boy Advance cartridge, and fits in the GBA slot on the DS & DS Lite. The Rumble Pak is incompatible with the new DSi as it lacks the GBA cartridge slot that previous models had.

The Rumble Pak was originally an add-on for the Nintendo 64 system, designed to fit into the Controller Pak slot on the system controller. The technology has also been incorporated into select Game Boy Color and Game Boy Advance cartridges before its current incarnation.
Source(s):
Rumble Pak (Wikipedia): http://en.wikipedia.org/wiki/Rumble_Pak

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