Sirius and Remus very carefully do not look at one another during this discussion of Snape's (non)toxicity, but at the same time, apart from acknowledging Q's presence, there's no obvious conversational opening.
"And here I thought," Remus says, just loudly enough for Sirius to hear, "that the infamous Black thumb was only found growing out of the palms of extremely inbred, but otherwise healthy, Black babies."
"I'll tell you where you're going to find it in a minute if you don't shut up right now," Sirius says, equally softly, and trying very hard not to smile.