Quirrell. Standing in front of the mirror, making frustrated gestures at it. Well, at least they'd already guessed it was probably Quirrell; that meant Percy didn't have to spare a moment for confusion. "Stupefy!" he hissed, as quietly as possible, pointing his wand at the Defense professor's back. What was it Professor Greengrass had said? Ninety percent of battles are won by ambushes, he'd said, when explaining why they needed to know how to defend themselves even if they weren't expecting to be attacked.