The odd wheel's unorthodox mode of travel exemplifies a recent trend: Physicists are finding ways to get useful collective behavior to spontaneously emerge in robots assembled from simple parts that obey simple rules. "I've been calling it robophysics," said Daniel Goldman, a physicist at the Georgia Institute of Technology. The problem of locomotion ?" one of the most elementary behaviors of living things ?" has long preoccupied biologists and engineers alike. When animals encounter obstacles and rugged terrain, we instinctively take these challenges in stride, but how we do this is not so simple. Engineers have struggled to build robots that won't collapse or lurch forward when navigating real-world environments, and they can't possibly