A vortex is a whirling mass of fluid such as seen in a tornado, a whirlpool, a smoke ring, or water running out of a bath. The term is used in hydrodynamics for a portion of fluid in which the individual particles have circular motions. A vortex may be produced when two adjacent fluid streams have different velocities or when a solid body moves through a fluid and vortex lines cannot begin or end inside the fluid.