As a child, you may have been fascinated to learn that draining the water from a bathtub causes a spinning tornado to appear.
That or gravity may have been your first introduction to classical mechanics. As the water rotated faster, a vortex appeared.
Yet if the water is extremely cold liquid helium, the fluid will swirl around an invisible line to form a vortex that obeys the laws of quantum mechanics. Sometimes, two of these quantum tornadoes flex into curved lines, cross over one another to form a letter X shape, swap ends, and then violently retract from one another—a process called reconnection.