Powder
"What are you two doing, now?" Ron asks, coming up behind Fred and George. Said twins are fighting over a small jar filled with what looks like white powder. They both turn on their heels and grin when they hear his voice, Fred claiming the jar for the moment.
"Trying to decide who gets to have the honor of pulling one over on Snape," Fred replies, stealthily moving the jar to his other hand and out of George's range.
"Yeah, he'll never-" George breaks off.
"-see this coming," Fred finishes. George silently waves his wand and the jar flies quickly into his hand, Fred muttering 'cheater' under his breath as it does. George grins to himself.
"What're you going to do?" Ron asks, eyeing the powder warily, but also excited about the prospect of a prank on Snape. "What is that stuff?"
"This, my dear brother-" Fred begins.
"-is nothing less than-" George continues.
"-the finest quality of-" Fred keeps going.
"-Peeves' Powder!" they shout in unison.
"The newest product in the-" George starts again.
"-Weasley's Wizard Wheezes line! Of course-" Fred follows.
"-Peeves' wanted us to name something after him, so-" George says.
"-we finally did!" Fred exclaims happily.
"But what does it do?" asks Ron, who honestly doesn't care what Peeves wanted. The twins glance at each other and grin.
"It sticks," they say in unison. Ron gives them an exasperated look.
"What?" George questions, seeing the look. "That's what it does!"
"Doesn't come off with magic, either," Fred explains. "Has to be-"
"-washed off-" George cuts in.
"-the muggle way. It's really a-" Fred says.
"-bother, and it takes some vigorous-" George continues.
"-scrubbing as well. Not just a-" Fred goes on.
"rinse will get this stuff off," George says.
"Glamour doesn't work, either!" Fred finishes.
"That's great, but why's it such a big deal you get Snape with it?" Ron asks, showing his sometimes clueless nature. It's the twins' turn to be exasperated.
"Have you seen his hair?" Fred asks, willing Ron to figure it out on his own. A look of realization comes over Ron's face.
"Wicked!" Ron exclaims, now grinning as well. "When're you going to pull it?"
"In a few minutes," George says.
"When he's coming into the Great Hall for dinner," Fred explains, and quickly snatches the jar from George. George looks about to tackle him to get it back.
"He's coming!" Ron whispers furiously, causing both twins to freeze and Ron to scurry away, not wanting to get blamed for being a part of the scheme. George quickly steps back into a nearby alcove, giving this one to Fred only because he has the jar. Fred gestures frantically, and George quickly does the levitating spell. Fred floats high to the ceiling of the entrance to the Great Hall, so as not to be seen. Snape stalks past, obviously not expecting anything to happen. But, why should he? He is the most feared teacher at Hogwarts, after all.
George suddenly lowers Fred quickly, making sure it's out of Snape's sight. 'Now!' George mouths, and Fred dumps the powder on Snape's head quickly. While Snape stands in part shock-part rage, George whizzes Fred away, running down the hall and turning the corner before stopping and dropping said floating twin. They grin at each other, both leaning against the wall. George pants for breath, having run down the corridor as if his life depended on it. Which, now considering, it probably did.
"That'll make the Greasy Git to wash his hair!" the twins exclaim in unison, only to feel a shadow descend upon them. Said shadow owner is in a silent rage, which the twins can tell just by the change in the air when it appeared. They both gulp, and Fred's the one to gather the courage to raise his head first, George following shortly after. Both grin at their potion's master's head and shoulders covered in their powder, knowing they can't get into any more trouble. Snape's scowl of rage – if possible – deepens more at their expressions. Both quickly frown and pale slightly when they remember who they're dealing with.
"Regretting it yet, George?" Fred questions quietly.
"A bit," George replies. "You?"
"A bit."