Quietly as possible, they crept along the next corridor after Snape's fading footsteps.

“He's heading for the third floor,” Harry said, but Ron held up his hand.

“Can you smell something?”

Harry sniffed and a foul stench reached his nostrils, a mixture of old socks and the kind of public toilet no one seems to clean.

And then they heard it — a low grunting, and the shuffling footfalls of gigantic feet. Ron pointed — at the end of a passage to the left, something huge was moving toward them. They shrank into the shadows and watched as it emerged into a patch of moonlight.

It was a horrible sight. Twelve feet tall, its skin was a dull, granite gray, its great lumpy body like a boulder with its small bald head perched on top like a coconut. It had short legs thick as tree trunks with flat, horny feet. The smell coming from it was incredible. It was holding a huge wooden club, which dragged along the floor because its arms were so long.

The troll stopped next to a doorway and peered inside. It waggled its long ears, making up its tiny mind, then slouched slowly into the room.

“The keys in the lock,” Harry muttered. “We could lock it in.”

“Good idea,” said Ron nervously.

They edged toward the open door, mouths dry, praying the troll wasn't about to come out of it. With one great leap, Harry managed to grab the key, slam the door, and lock it.

“Yes!”

Flushed with their victory, they started to run back up the passage, but as they reached the corner they heard something that made their hearts stop — a high, petrified scream — and it was coming from the chamber they'd just chained up.

“Oh, no,” said Ron, pale as the Bloody Baron.

“It's the girls’ bathroom!” Harry gasped.

“Hermione!” they said together.

It was the last thing they wanted to do, but what choice did they have? Wheeling around, they sprinted back to the door and turned the key, fumbling in their panic. Harry pulled the door open and they ran inside.