The flagellum, the organelle that enables bacteria to swim, consists of a long, thin filament that is rotated at hundreds of revolutions per second by a motor embedded in the cell surface. The filament is essentially a tube made up of 11 protofilaments arranged as a helical supercoil. By switching between left- and right-handed helical twists (the L and R states, respectively), a bacterium can switch between 'running' and 'tumbling' (reorientating) motions. In the 15 March Nature, a team led by Keiichi Namba of the Protonic NanoMachine Project and Matsushita Electric Industrial Company, Kyoto, Japan, provides an insight into how filaments are able to switch helical states (Nature 2001, 410:331-337).