(There was an idea much beloved and written about by this country’s philosophers that magic had to do with negotiating the balance between earth and air and water; which is to say that things with legs or wings were out of balance with their earth element by walking around on feet or, worse, flying above the earth in the thin substance of air, obviously entirely unsuitable for the support of solid flesh. The momentum all this inappropriate motion set up in their liquid element unbalanced them further. Spirit, in this system, was equated with the fourth element, fire. All this was generally felt to be a load of rubbish among the people who had to work in the ordinary world for a living, unlike philosophers living in academies. But it was true that a favourite magical trick at fetes was for theatrically-minded fairies to throw bits of chaff or seed-pods or conkers in the air and turn them into things before they struck the ground and that the trick worked better if the bits of chaff or seed-pods or conkers were wet.)Slower creatures were less susceptible to the whims of wild magic than faster creatures and creatures that flew were the most susceptible of all. Every sparrow had a delicious memory of having once been a hawk and while magic didn’t take much interest in caterpillars, butterflies spent so much time being magicked that it was a rare event to see ordinary butterflies without at least an extra set of wings or a few extra frills and iridescences, or bodies like tiny human beings dressed in flower petals. (Fish, which flew through that most dangerous element, water, were believed not to exist. Fishy-looking beings in pools and streams were either hallucinations or other things under some kind of spell and interfering with, catching, or—most especially—eating fish was strictly forbidden. All swimming was considered magical. Animals seen doing it were assumed to be favourites of a local water-sprite or dangerously insane; humans never tried.)'s End
Robin McKinley