"Oh," Hermione replies, though the concept is, at best, an abstract one to her. "Is it a quantifiable sort of thing? A magical energy you can feel versus your body's normal energy? Maybe that's something I should take into account while practicing my spells here. We don't really have that back home; some spells are harder than others, of course, and practicing the more difficult ones for too long might wear a person out, but it isn't something we think of as anything other than simple exhaustion."