"Thought you might. Okay, so the theory is called Lialenan matter theory and it supposes that shapeshifting involves storing our alternative shapes in a semi-dormant state in an attached world-of-sorts that I named Lialen for mythology reasons. Right now I'm in my human form, but if I stay in it for a year and never turn back to my natural one in that time, I will find after that year that my natural shape has grown and healed from any injuries it had and so on. If I tuck away physical objects - like, if I shifted now, my clothes would go away with my human shape, they wouldn't be destroyed or anything - then they'll come back when I resume the form. Etcetera. Shifting is truly instantaneous once the intention to do it is formed correctly, like teleporting or summoning is when the spell is completed and unlike any spell that actually changes a thing itself - a color change takes a tiny fraction of a split to happen, but it still takes that period of time, for instance."