Severus stopped, frowning, when the beetles were only coarsely ground. They needed to be medium fine, so that wasn't what halted him in his grinding. Somewhere along the way, Linton's eyes had taken on a rather glazed look, and he was nodding distractedly, as if he was no longer listening. Either that, or he was listening, but he still didn't have the foggiest notion of what Severus was on about.
"You look confused," Severus said, still frowning, as he set down the mortar and pestle on the table. "Where did I lose you? Was it the cauldron conversions, or before that?"