Fill a bathtub with water and sprinkle baby powder liberally over the water’s surface to aid in viewing the motion of the surface water. Then take a spatula, insert it gently into the water, move it slowly and briefly perpendicular to its flat face, then extract it gently from the water. Twin vortices will have been generated. Observe the vortices’ motions. Explain 

(i) The generation of the vortices

(ii) The sense in which the velocity field of each vortex convects the other vortex through the ambient water. Use your bathtub or a swimming pool to perform other experiments on the generation and propagation of vortices.