It's not until the third day that Voldemort realizes what's happening. He should have seen it coming sooner, the way he smoothly overtakes Quirrell's mouth when a student asks a particularly challenging question, the way he chastises Quirrell for poor lesson plans and lack of preparation, the way he feels almost eager to present interesting findings to the students during the next class period…
So, yes, it might take three days, but Voldemort is not stupid.
He is the Defense Against the Dark Arts teacher at Hogwarts. He'd applied for the role twice only to be rejected both times—once by Dippet, who said he was too young, and once by Dumbledore, who despised him—and had resigned himself to never having it…but, here he is. Perhaps not technically the DADA professor, that honor still goes to Quirrell, but Voldemort is still effectively the instructor. The mere thought fills him with an emotion he finds vaguely familiar, one that he can't quite otherwise name.
On the fourth day, he resolutely shoves Quirrell into unconsciousness and takes over the classes entirely. It's a dangerous move—Voldemort is still weak and the students are bound to notice a change in his demeanor, no matter how much he stutters and quails—but he hopes that any noticeable exhaustion or changes in mood will be chalked up to trauma (which Quirrell certainly has enough of).
The students are immature brats, of course. Many did not do the homework. Some talk back to him. Some giggle at his stutter. For every three students that respect him, there are five that don't. Something inside Voldemort knows that he should be upset by it all, but…he can't bring himself to care because it's all overshadowed by a single incident.
Hermione Granger, an arrogant and slightly annoying Gryffindor first-year, raises her hand and asks a question, one advanced enough to stop Voldemort in his tracks. He stares at her in amazement and dives into a complicated explanation, watching Granger absorb his words like a sponge, and then her eyes light up in understanding.
Another vaguely familiar emotion bubbles up inside of him at that moment, but this time he recognizes it: it's pride—not for himself, but for her.
After that…well, it's not like he stops searching for the Philosopher's Stone, he just lets the hunt sit on the back burner for a bit. After all, he has lessons to plan.
A/N (posted 2022-10-29 on ao3): this is complete and utter crack. dont expect much lol