Katherine did not like not knowing things. In fact she prided herself on how much she did know. From a very young age, she'd had the very finest of tutors teaching her everything she could possibly want to know, from reading and writing and maths, to magical history and theory, to the art of potion making, broom riding as well as horse back riding. She was well versed in magical culture, both classical and popular (the latter mostly on her own accord) and knew all about how to move and interact in a social setting. In short she had a very extensive education and almost exclusively the highest grades. Which was why when faced with an unknown substance she was absolutely disgusted at that lack of knowledge (not to mention utterly paranoid over why there had been nothing in the Slytherin showers after her conversation with Professor Black yesterday).
When Professor Black's note arrived, her worry and annoyance with herself turned into absolute anger at the ones causing this and a determination for retaliation. She memorised the hex, the motion, everything and smirked when the note burned. The rest of the morning was spent following instructions to the letter, and enjoying every moment of it.