Snape drew a deep breath and bit back the first - and the hundred-and-first - sharp retort that came instantly to mind.
"For your information, it is considered impolite," Snape declared in slightly-too-airy tones that suggested a volatile brew about to explode, "to speculate on the sexual activities of anyone you are not sexually involved with."
Fuck me sideways with a rusty saw, Snape thought incredulously, and here I thought I'd done enough teaching.