He caught himself and erased the smile from his face as she mentioned the Potions classroom where he'd found it. He'd still been visiting it since those wretched events the year prior. But then, Lily surely wouldn't assume such things because she likely didn't know just how much he held onto those memories.
Trying to compose himself after the lapse, he stood up a little taller and stiffer, shouldering his heavy bag as a sort of shield between him and Lily. "I'm surprised. I wouldn't have thought you to be so sentimental about old memories like those. They never seemed as important to you as —" Don't say 'to me', don't, his mind urged, and he shut his mouth before it blurted out.
Severus turned away and watched some of the straggling students already heading in various directions to get ready for classes. The tension between them rose entirely of his own doing and he just wanted to join them in the crowd for once.
"I suppose I should head to class. I need to study for the upcoming exam in Charms..." he trailed lamely, barely passing her another glance as he itched to get away.