Interactive designer Lauren McCarthy used Arduino to make a wooly
torture device hat that she calls the Happiness Hat — it has sensors that detect when you're smiling — when you're not smiling, it sticks a metal spike into your head. Here's a short description from her web site:
An enclosed bend sensor attaches to the cheek and measures smile size, a servo motor moves a metal spike into the head inversely proportional to the degree of smile. Through repeated use of this conditioning device you can train your brain to smile all the time.