It began, as most pranks went, with a bang.
It was their first year at Hogwarts, and Fred and George Weasley were in detention yet again. The two red-headed mischief makers were sitting in Binns's classroom after dinner, having been assigned cleaning duty and over two hundred lines each of "I will not give Mrs. Norris catnip-laced mice, nor am I to tell Mr. Filch that the mice were an early Christmas gift from the First Years...". There was a sudden crash from a hallway several floors down below, and Binns' translucent head peeked out of the chalkboard up front in order in order to better hear the noise. Fred grinned; it appeared that Lee Jordan had fulfilled his promise to pull pranks to fill in for them while they were in detention, and from the sudden rapid footfalls outside the classroom door, he and his brother were about to get credit for it.
True to prediction, the familiar severe gaze of an annoyed Professor McGonagall appeared at the doorway, followed by a sheepish-looking, soot-covered Lee Jordan, who was holding what appeared to be the detonated remnants of a rather sizable Filbuster's Firework, still belching greenish-grey smoke and the occasional purple spark. "I don't know how, I don't know when, but I do know that one or more of you two, Mr. Weasley and Mr. Weasley, are at least partially responsible for this. What in the name of Merlin possessed you two to help Mr. Jordan set off fireworks in the Charms corridors?"
George looked at Fred, who nodded in silent agreement. The two promptly began defending themselves.
"It wasn't us, Professor, see, it was Peeves, he'd got hold of some Filbuster's Fireworks from Filch's cabinet-"
"And then he set 'em off in the corridors, see-"
"But some went off too early, and then BOOM! the whole thing blows up all over the place-"
"And you know how this old castle is, all those high ceilings and big rooms help make everything sound worse than it really is-"
"Lee was heading back to the dorms with us after dinner, and you know how the Charms corridor's a shorter route back to the Common Room-"
"But Peeve's had gone and set off all the fireworks, the smoke was so thick you couldn't even see your hand in front of your face-"
"And when Filch'd gone to check out all the noise, the smoke had cleared and Peeves, he knew the horrible racket Filch'd make when he saw the mess, so when Lee showed up, he used all of us as patsies to pin the whole thing on! We're innocent this time!"
Not out of breath whatsoever from their rapid back-and-forth speech, both boys stared expectantly at Professor McGonagall, who pursed her lips into a thin line and said calmly, "So, then, you two are willing to hold out your hands so that I may check for any possible fireworks residue? Surely you don't believe I am that gullible as to let you two off without at least a proper glance over..."
Both boys looked at each other for a second, contemplating their impending fate, before George held out his hands first, clearly covered in black soot. Fred, however, was shaking something behind his back. After McGonagall had looked over George, she turned to Fred, gazing at him with an expectant look.
Fred threw the Filbuster's Firework into the air, pointing his wand at the fuse and shouting "Incendio!"
The fuse ignited, rushing down the string, and the firework exploded. McGonagall banished the explosion instantly, though she wasn't quite quick enough to keep from being coated in soot residue, painting her face, hands, and the front of her robes black. With fire in her eyes, she whirled around, ready to march both twins to the Headmaster's Office...but they weren't there.
Tearing off down the corridor and back to their Common Room, Fred and George ran like the hounds of Hell were after them. In essence, they had something much, much worse coming after them: their very angry Head of House, who, as a Transfiguration Professor, was certainly more than capable of making a creative punishment for them.
As they sped past the nearest suit of armor, Fred high-fived his twin, grinning widely. "Look at it this way, Forge old boy, at least she hasn't found out about the toilet we blew up this morning."
George mirrored his brother's happy expression. "Quite right, Gred. Let's send it to Ginny, Mum'll have a fit. We can go to the Owlery and borrow a school owl, Errol's too old to carry something that heavy...".
Elsewhere in the castle, Moaning Myrtle was having an unusually good day. A toilet that had blown up in her bathroom had soaked a bunch of First Year girls, causing them to run out screaming for help getting the unmentionable gunk off. She'd have to thank those twins someday, they were the only boys who'd ever come into her bathroom so far who'd done something worth laughing herself all the way up to the ceiling about.