A swarm of robots inspired by living cells can squeeze through gaps and keep moving even if many of its parts fail. Living cells gather together and collectively migrate under certain conditions, such as when inflammatory cells travel through the bloodstream to a wound site to help the healing process. To mimic this, Hod Lipson at Columbia University in New York and his colleagues created 25 disc-shaped robots that can join together. Each is equipped with cogs that cause the robot's outer shell to expand and contract and magnets around its perimeter that let it stick to neighbouring bots. Individually, the bots can't move, but once stuck together, the swarm can slither across a surface by making individual bots expand and contract at different times.