In college courses on applied math (and possibly physics) there is a topic called “Fluid Mechanics,” widely–but falsely–regarded a a soft elective because it is all governed by just one equation (given a plural here, but basically just one). It’s the study of how fluids move ( e.g. oil through pipelines) and how objects move through fluids (e.g. planes, submarines). OK, got the following from a reader who knows his fluid mechanics:

Mr Derbyshire—The following can be found with attribution in Mandelbrot’s Les Objets Fractal:
Little whorls have littler whorls
Increasing with velocity.
And littler whorls have littler whorls
And so on till viscosity.
(Of course a take-off on Swift’s ‘Little fleas have littler fleas / Upon their backs to bite ‘em / And littler fleas have littler fleas / And so ad infinitum.’)