She would try to alter one of her old robes

Hermione made her way into the transfiguration classroom.

She had read in the tome of robe alterations that the transformation spell needed to expand the size of her old robes required certain materials. Since most spells only required a combination of wand-waving, reciting the words, concentration, and intent, this spell was rather new to her.

Hermione began to dig through the supply cupboards, acutely aware of the draft caressing her undercarriage as she bent over to dig through the drawers, searching for the thread McGonagall had her first-year students turn into hairs.

When at last she had found her two spools of thread, red and black. She stood up.

She started to get chills, but having masturbated so recently, she decided that it might be better for her to hold off for now. But she'd want to come back to the transfiguration room later... Just to pretend that McGonagall was using her as a live model for human transfiguration...

After Hermione reached the Fat Lady, she was stopped before she could sit down in front of the portrait.

"I'm not letting you in!" The Fat Lady said.

"But... what?! We had a deal!" Hermione protested.

"I wouldn't let you in, even if you were dressed right now!" The Fat Lady retorted. "Not until you tell me all about what happened last night."

After she had said that, Violet dashed into the picture, out of breath, holding her hat in place.

Hermione sighed. She told them about how they made her fly naked defending the goalposts, how she wooshed around the field to throw quaffles past Daphne, how she managed to hit the Slytherin with a bludger, how she only just barely caught the snitch in time, and how she orgasmed right as she caught it.

Violet and the Fat Lady giggled and kept moving their heads from looking at each other to looking at Hermione. Hermione had to admit that recounting that story was turning her on, but she wanted to wait to play with herself until after she had gotten in the safety of her own room.

"Well... It sounds like if you had spent more time practicing and less time with your nose in a book, Gryffindor's team might have been unbeatable!" The Fat Lady said after Hermione finished her story.

"Especially," Violet interjected, "if you played with the uniform you're wearing now."

Hermione blushed as she sat down and spread her pussy open. The two women laughed as the portrait swung open, allowing Hermione inside again.

She ran up to her bed as fast as her legs would take her and was about to move her fingers to her slit when she had an idea. She bounded from the bed and began searching... At last, she found it!

One of Fred and George's simpler inventions was a simple doorknob with a cord attached to it. The idea was that if you locked the door, then someone coming in could twist and pull the knob, only to reveal the string attached to it. The doorknob used to say plenty of rude words when it was loose, but after Mrs. Weasley found the thing, she was incensed and did everything she could to make it shut up, but nothing had worked.

Eventually, Hermione agreed to take it away herself to undo the charms put on it. Once she had gotten to Hogwarts, she went straight to the library to learn the spells needed, and now it was just a doorknob on a string, safe enough to drop in the middle of a muggle town, even it would confuse them.

Hermione picked up her wand. "Reducio!" She shrunk the doorknob down to a third of its size. "Celerquasseo!" The doorknob began to vibrate rapidly. Taking some of her own fluids and rubbing them all over the doorknob, she inserted it inside of herself, letting it rest inside of her with the string poking out.

This was wonderful! It felt fantastic! She had never felt this sensual before! She decided to descend the stairs from her bedroom into the common room below. She strolled through the room, naked as could be, with her magical vibrator inside of her. She strutted, testing the lengths of her footsteps, but the device did not leave her pussy! After a few minutes, she came right in front of the great fireplace, and the device was still there. At last, she tugged on the string, and it slid right out again.

She'd have to try to use this later in the main castle, but for now, she returned to the reason she had gathered the thread.

She went back up to her bedroom, deposited her new toy into her bag after cleaning it, and dug through her trunk again, pulling out one of her old robes. These robes had been with her since her first year, but she had them altered back before her third year. As she measured it against herself, she saw that, while she could theoretically fit in it, it was rather small. Why, the hem came up to only a few centimeters below her pussy! At least the chest would still fit well, even if the sleeves were short.

She took out the thread and began to wave her wand around, speaking the incantation. The thread spun through the air before making its way to her robe. It multiplied and weaved itself around, moving old threads aside as it moved in to expand the fabric in the areas where it needed to be.

After a while, Hermione looked at her handiwork. She had surprised herself, as this robe looked no worse for wear than one of Ginny's secondhand robes! She put it on and it felt perfect on her body. It came down past her ankles, just above the bottoms of her feet, the sleeves were just the right length, and she could breathe well in it with no constriction of the chest.

There was still quite some time before she'd have to go to dinner, but she had already finished her homework for the holidays. Hermione pondered on how to pass the time for now...