Карусель Гельмгольца 1

Two spherical Helmholtz resonators (glass Christmas tree bulb ornaments) connected by a thin rod and suspended from a thread will rotate when driven at their resonance frequency. The cause of rotation is due to a nonzero time average of the momentum flow into and out of the opening of the resonator. Asymmetry in the opening of the resonator causes flow separation and vortex rings that produce a reaction force (and hence a torque) on the system, causing it to rotate.