Okay, needed for this experiment are:
1. You
2. A *completely* dark room
3. A light switch in the *completely* dark room (working, preferably ;) )
Okay, turn the light(s) off and let your eyes get accustomed to the dark for a couple minutes. Now, flip the light on and focus on something for 2-5 seconds (preferably nothing near the light itself), then shut the light off again. Don't change your gaze, and watch the after-image appear. SO COOL :D