Sadde was counting on that! Apparently the instinct is strong even amongst wixen.

...just to have some fun (it is her third day, she's allowed to have fun now, right?), the next time she catches one of the kids' eye without Madam Pomfrey's attention she morphs her tongue into a lizard's and does the little tongue-flick thing lizards tend to do. Just for a fraction of a second, then her tongue's hers again.