What are the best two legs for running an obstacle course? One leg that crawls at the knee joint, and one massive leg dragged behind for stability like a kangaroo's tail, according to a recent simulation. David Ha, a researcher at Google, created a virtual robot with a wide head supported by two legs and tasked it with crossing a randomly generated landscape within a time limit. It learnt to do this with an algorithm used in artificial intelligence called reinforcement learning. When the terrain is fairly flat, the AI crossed most quickly when it developed a jaunty skipping gait that it performed on the'knees' of its long, skinny legs.