“I wonder if Hermione’s seen this yet?” Harry said, looking around at the door to the girls’ dormitories.

“Let’s go and tell her,” said Ron.  He bounded forward, pulled open the door, and set off up the spiral staircase.

He was on the sixth stair when it happened.  There was a loud, wailing, klaxonlike sound and the steps melted together to make a long, smooth stone slide.  There was a brief moment when Ron tried to keep running, arms working madly like windmills, then he toppled over backward and shot down the newly created slide, coming to rest on his back at Harry’s feet.

“Er — I don’t think we’re allowed in the girls’ dormitories,” said Harry, pulling Ron to his feet and trying not to laugh.

Two fourth-year girls came zooming gleefully down the stone slide.

“Oooh, who tried to get upstairs?” they giggled happily, leaping to their feet and ogling Harry and Ron.

“Me,” said Ron, who was still rather disheveled.  “I didn’t realize that would happen.  It’s not fair!” he added to Harry, as the girls headed off for the portrait hole, still giggling madly.  “Hermione’s allowed in our dormitory, how come we’re not allowed — ?”

“Well, it’s an old fashioned rule,” said Hermione, who had just slid neatly onto a rug in front of them and was now getting to her feet, “but it says in Hogwarts, A History that the founders thought boys were less trustworthy than girls.”

Harry Potter and the Order of the Phoenix – J.K. Rowling