Harry thinks it's his dad producing a patronus and saving him and Sirius. It's actually him going back in time and doing it himself.
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One of the best scenes in the whole series, Hermione punches Draco in the face after threatening him with a spell. Draco cowers and runs away - classic!
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Harry Potter and the Prisoner of Azkaban Quotes

[to Professor Snape] For in dreams, we enter a world that is entirely our own. Let him swim in the deepest ocean or glide over the highest cloud.

Dumbledore

Hermione: [after Hagrid gives Ron Scabbers back] I think you owe someone an apology.
Ron: Right. Next time I see Crookshanks, I'll let him know.
Hermione: [annoyed] I meant me!