This Demonstration shows streamlines of the flow of a uniform, incompressible liquid past a rotating solid cylinder. Streamlines are imaginary lines drawn to illustrate the motion of the liquid; they can be interpreted as trajectories of small, light particles moving with the flow. Depending on the rotation speed, there are one or two stagnation points, displayed as red dots, where the liquid has zero flow velocity. An arrow indicates the magnitude and direction of the cylinder's rotation.