A small idea that I got after reading Eyes of a Tiger, a good Snape/OC fanfic by thevamyrpe13. Enjoy!
Harry smirked at Ron, who was bottling up the unused Polyjuice Potion, left over from their "We–thought-that-Draco-was-the-heir-of-Slytherin-but-he's-not" expedition. It was payback time.
Snape had been getting on their nerves recently, more than normal that is. What with his prowling after Harry, no doubt to catch him at some wrongdoing, and stuff, the snarky Potions master was on the dynamic duo's last nerves.
"Alright, now all we have to do is get a hair from Snape." Harry said to Ron. They were planning on transforming into Snape, then would spend the hour being extremely nice to everyone. They even had sweets to hand out!
"How are we going to do that?" Ron moaned. Harry pondered this for a bit.
"We could sneak into his rooms and grab a few hairs from his comb." Harry finally said.
Ron nodded. "You're right, we have no chance against him if we engaged him in battle."
They snuck down to the dungeons and picked the lock to Snape's personal quarters. The two young boys jumped at the slightest sound.
"Harry." Ron whimpered, tugging on his friend's robes. "I don't want to do this anymore, let's go back."
Harry shrugged Ron off. "Go then, I'll..." But he stopped talking as he found it. He darted over to the silver-backed hairbrush that was lying on a side table and picked it up. Ron stared at the object in Harry's hand as they hurried out of Snape's rooms.
"That hairbrush looks really feminine..." the red-head thought. "Oh well. It's Snape, he's an odd guy." He mentally shrugged.
Once in the safety of their dorms, Harry dished up two serving of Polyjuice Potion. "Now, Ron, remember that you should wait fifty-eight minutes until taking your dose, so we can have two friendly-Snape hours." Harry reminded Ron, who nodded.
Harry took a deep breath and added the hair, a strangely long and silky one. The potion turned a light black, if that was possible. It was like... the black of the night sky on a cloudless night at midnight.
"How is it," said Ron, "that Snape looks better than Crabbe or Goyle?" Harry ignored this statement. The Boy-Who-Is-About-To-Make-A-Fool-Of-Himself took a deep breath and downed the potion. The glass shattered on the floor as Harry doubled up, his hair shooting out of his skull, shooting up. Harry finally straightened up and looked at Ron, who was staring at him in horror.
"How do I look?" Harry spread his hands out.
"... like a girl." Ron said after a long pause.
"WHAT?" Harry screamed.
"You look like Professor Sinistra, mate." Ron started laughing as Harry ran to the mirror to check that he was, indeed, the Astronomy professor. Unfortunately for him, he was. The boys stayed in their dorm for the next hour, waiting for the potion to wear off. Harry just sat and glared at nothing, and Ron kept giggling when he glanced at Harry, who would just glare harder, which made Ron laugh more. It was a vicious cycle.
In the end, Harry and Ron snuck up the Astronomy Tower, into Sinistra's chambers, and put the comb on her desk, so that she would see it when she came back from her lesson.
Aurora Sinistra did see the comb when she got back, although it was from a date with her boyfriend, not from a lesson. She picked it up, frowning.
"What the heck?" she asked herself. "I could have sworn I left this comb in Severus' chambers..." She shrugged, making a note to herself to bring it back down that night after her lessons.