I'm going to risk talking about something that I've considered a bit...provocative. Too outlandish for airing even in a relatively quiet Internet backwater like Cohost. But perhaps I should...steam ahead with this train of thought. It's been revolving in the back of my mind for a long while.
It boils down to this: strong hints have been dropped within the Pnictogen Wing that if we ever wanted to see if human magical spells could have a measurable physical effect, something that could be studied empirically, then THIS is the one we should try to master.
Every Fate/stay night fan knows this one: Trace On. It was the one spell which Emiya Kiritsugu was willing to teach his adoptive son Shirou how to use, because Kiritsugu was thoroughly done with magic at that point. He considered it so shameful, it was the first thing he ever confessed to Shirou when Kiritsugu first visited him in hospital: "I need to tell you, I'm a mage." It's definitely a sign that, in the mundane world of Fuyuki City anyway, it's not great to admit you're a mage. Kiritsugu needed to get that out of the way immediately.
I have wondered how difficult it must have been for Emiya Kiritsugu to yield to Shirou's requests—I imagine they must have been frequent and insistent, for surely Shirou's belief in his father's heroism was cemented in Shirou's mind by Kiritsugu's mysterious past as a mage—to teach him one spell, when surely the very idea of ever casting a spell again filled him with revulsion and bitterness. That must have been a tough lesson to teach, but Kiritsugu did...and it saved the world.
"Trace On" fascinates me, because it's a spell I can very nearly understand. With our substantial knowledge of chemistry and the structures of ordinary substances, we can dimly glimpse the subtle rearrangements and realignments that might be going on when Emiya Shirou casts "Trace On" with a piece of wood or a sheet of paper or whatever. Imagine temporarily increasing the crystallinity of the cellulose in paper, for example, aligning the long chains a bit. There's wiggle room in there; these are flexible, supple molecules. "Trace On", I think, could actually work. Nasu came up with a real winner there. It makes perfect sense as a beginner spell precisely because it's relatively easy to visualize in physical terms.
And...it would be testable.
Just sayin'.
~Chara of Pnictogen