When Sean was a little boy, he and I took a cigar box, some powered wheels, a coat hanger and a superball and built a robot with one bit of processing power. Not byte -- bit. I was pretty proud of that, and figured that we'd just set a record that would last forever.
I was wrong.
A bristlebot is a robot with zero processing power. It's made by taping the motor from a pager and a watch battery to the back of a severed toothbrush head. Let it go and it skitters around randomly. So far, pure motion, no robotic behavior.
When placed in a confined space, the bristlebots continue to move about randomly. However, when they take up at least 8 percent of the space's area, swarming behavior emerges.
The above video explains it all. But if you want to cut to the emergent behavior, skip ahead to 1:27.