- How could a troll get in?
- Not on its own. Trolls are really stupid. Probably people playing jokes.

9 seconds sound clip from the Harry Potter and the Sorcerer's Stone movie soundboard.

You can hear this line at 01:12:46 in the Blu-ray version of the movie.

Quote context

[...]

- Silence!

- Everyone will please not panic!

- Now... prefects will lead their house back to the dormitories.

- Teachers will follow me to the dungeons.

- Gryffindors, keep up, please, and stay alert.

- How could a troll get in?

- Not on its own. Trolls are really stupid. Probably people playing jokes.

- What?

- Hermione! She doesn't know.

- I think the troll's left the dungeon.

- It's going into the girl's bathroom.

- Hermione, move!

[...]