As Snape started explaining human nature to Hermione, he found himself warming to the topic. He hadn't had a chance to lecture to a captive audience about anything besides potions for a while, and Hermione was the one first year he could plausibly believe would be capable of understanding any kind of sophisticated lecture. Although she didn't ask any questions, she seemed to be following along as he explained why one should never do an enemy a small injury. He found himself rambling a little bit, and finally cut himself off when he started to tell an anecdote that had ended with Barty Crouch tortured near to the point of insanity. He cleared his throat and made sure he had her full attention.