Simon was not having the best of days. He had these little episodes sometimes, never anything he wanted to worry Amelia or Edgar with -- if they'd even care, a tiny, malicious part of his brain whispered -- and they were distressing when they happened, even if nothing actually bad went on. Severus's suggestion of an extra calming draught had been a welcome one, one that Simon probably wouldn't have thought of himself, and he'd spent the intervening time pacing, practically wearing a path through the worn carpet.
At the knock on the door, he jumped a little, startled, before remembering that Severus had said he'd bring the potions rather than owling them. Shoving his glasses back into place and trying not to inhale too deeply -- the air still tasted like poison, even if it didn't smell like it -- he went over to the door and opened it, giving Severus a slightly jittery, awkward smile.