Helicity:
Imagine the tiny tornado that forms in your tea as you stir it. The swirl can have three parts: a link, in which one loop passes through the centre of another, like chain links; a writhe, in which a loop gets a kink in it like an unruly garden hose; and a twist, when several streams of water flow around one another, like the strings in a twisted rope.
These three motions combine in a vortex to make up what’s called its helicity … [“Teacup tornadoes brew up a storm,” NewScientist (12 August 2017)]
