Perhaps the most advanced tagging concept is "smart dust," clouds of "motes," tiny micro-electromechanical sensors that can attach themselves to people or vehicles. Thousands of these sensors would be scattered at a time to increase the chance of at least one of them reaching its target. Kris Pister, a professor at the University of California at Berkeley, was sponsored by the Defense Advanced Research Projects Agency (Darpa), the Pentagon's R&D branch, more than a decade ago to work on smart dust and was able to create sensors the size of rice grains. In the beginning, he now says, he and his colleagues imagined "smart burrs" that could attach to a target's clothing as he or she brushed by, or "smart fleas" that could jump onto their targets. Pister says that this kind of autonomous microsensor is probably still not feasible. In 2001, however, his group succeeded in scattering more-primitive smart-dust motes from a small aerial drone and using them to track vehicles. A single UAV could easily carry thousands of tags, he says.