This cool Instructable teaches you how to build your own persistance of vision wristband, one of those things that spell a word when you swing it in front of someone fast enough. The device uses a Lilypad Arduino board and about eight 8 LEDs.

You actually have to program the board to get things to show up, which requires a bit of patience. However, it’s quite a cute illusion and, with enough patience, you can make your wristband say anything, if you catch my meaning.