The next day, when Snape swept into the fourth-year Defense Against the Dark Arts classroom, he immediately learned what Minerva meant by "rampant."
Before he even reached the door, the keen sense of smell he had honed through years of dealing with dangerous ingredients sounded an alarm.
Almost at the same moment, there was a soft pop from the floor tiles a few inches in front of his toes, and a cloud of yellowish-green smoke with an indescribably foul stench exploded outward.
Snape's reaction was astonishingly quick. With a sweep of his black robes, he glided two steps back, while silent Cleaning and Ventilation Charms had already flown from his wand, dispersing the source of the stink along with most of the smell.
Yet a trace of stubborn lingering odour still wormed its way into the classroom and refused to dissipate.
Suppressed laughter came from inside, especially from the two conspicuous redheads in the back row.
Snape's expression darkened at once, enough to drip venom.
He strode into the classroom, the wind stirred by his black robes carrying a chill as his gaze viciously raked over George and Fred Weasley.
"Gryffindor," he said in a low, icy voice, "twenty points will be deducted for openly setting up prank magic in the corridor and attempting to attack a professor."
He paused, then added, "If anyone feels that the air in this classroom still needs 'improving,' I would be delighted to let them experience the more 'lasting' fragrances in Mr Filch's private collection."
The classroom instantly fell silent.
The Weasley twins curled their lips, but did not dare make another sound.
Snape announced his arrival in the cruellest possible fashion: there would be no jokes here, only discipline and consequences.
Suppressing his anger—and a trace of incomprehension at how Lin Du had