"Potter? Are you feeling unwell?" Snape asked, in a voice of faux concern.
"I'm fine, sir." Harry Potter responded.
"If the ferocious celebration occurring in Gryffindor is not giving you a headache..." Snape said, letting his voice trail off ominously, "Why are you here?"
Abruptly, Harry Potter pulled the marble out of his pocket. He lifted it, still in the palm of his hand. It glimmered in the low light cast from the torches. Why were there torches? "What is this?" Harry asked, his eyes flicking up at last to Snape's, "Don't tell me you merely acquired a curiosity, and decided it was an excellent gift." Harry's sternness melted into a more childlike questioning, "What does it do?"
"All this, and without a thank you," Snape growled, gratified to see the dumbfounded look on Potter's face. "Or a sir, for that matter." The dawning look of horror was almost worth the insubordination. This time.
"Sir!" Harry Potter said crisply. "Please, sir!" This was delivered in about the opposite of a pleading tone, the demand of a puppy that just couldn't see anyone not giving it what it wanted.
In this case, the boy was right.
Snape stood, rounded the desk and plucked the marble out of Potter's hand. "This is a trick. A very particular trick, in fact." Snape began to roll it over his palm, with slight tilts of his hand to give it momentum via gravitation.
"What does it do?" Harry Potter asked, his voice soft as a whisper, and eager as an unstretched hound*.
"It distracts." Snape closed his hand around the marble - and threw it at the wall. It hit, and bounced, and was on its third bounce before Potter caught it.
As Harry Potter looked up in triumph, he caught Snape's eyeroll. Looking more abashed, the boy straightened up.
"I did not mean for it to distract you in the here and now." Snape said, his comments, as usual, withering.
"Sorry, sir." Potter said.
"What is the worst danger when you use that ridiculous invisibility cloak of yours?" Snape asked, for a wonder waiting for an answer.
"Overconfidence, sir." Potter said, elaborating, "It does not hide my footsteps, nor will it shield a suit of armor from my weight. My smell is unchanged, as well, sir."
"Indeed," Snape said, his eyes sparkling, as he plucked the marble out of Harry's hand. "This is a trick, one I've used often enough, when those who search for you draw too close for comfort."
"A distraction..." Harry Potter said, looking at the marble in Snape's fingertips.
"A sound, a repeating sound that drifts away from your position. The ball itself is small enough to be missed." Snape said, with a crisp nod.
"Thank you, sir," Potter said with a dawning grin.
"Not so fast." Snape's hand clasped on Potter's shoulder like a vise. "There are more instructions."
"It would be wisest for you to use this in situations where you Will Not Be Caught, particularly by Aurors." Snape's eyes seemed to burn as they glared down at Potter. "The Aurors know this trick for mine. If they catch you using it, there will be Questions." Snape's eyes seemed to brighten, "Of course, Auror Custody would serve as a terrific way to keep you out of trouble."
"Thank you, sir!" Harry Potter said, cupping his hands until Snape dropped the marble into them.
"Get back to Gryffindor before people notice you've absconded from your duty." Snape snapped.
"Yes, sir!" Harry Potter responded, his crispness entirely spoiled by his broadening grin.
*This term is a neologism. It means a hound kept in the kennels, that wants to get out and stretch his muscles.
[a/n: Leave a review? I love how even Snape's office door is terrifying.]