Universal Laws
But magic is never as simple as people think. It has to obey certain universal laws. And one is that, no matter how hard a thing is to do, once it has been done it'll become a whole lot easier and will therefore be done a lot. A huge mountain might be scaled by strong men only after many centuries of failed attempts, but a few decades later grandmothers will be strolling up it for tea 'and then wandering back afterwards to see where they left their glasses.
--Maskerade by Terry Pratchett