"Oh," he says, surprised. It make senses, but he is still surprised. "Really?" He gives Rus a no-fair-when-I-can't-play-back grin, and goes back to his spelling game, pausing between the letters in case Rus wants to chase his darts.
Severus looks speculatively at Rus's feet. There's a chance there might be an extra stomach down there, after all. He looks up at the list, looking faintly nauseated. "Excellent question."
Ick. :( Better get a sample of the same stuff, unfortunately, and find out whether it turned or you got allergic to something; that can happen.