Book Quote by Joanne K. Rowling from Harry Potter - and the Half-Blood Prince

"'Do you remember me telling you we are practicing non-verbal spells, Potter?'
'Yes,' said Harry stiffly.
'Yes, sir.'
'There's no need to call me 'sir' Professor.'
The words had escaped him before he knew what he was saying."

Book Quote by Joanne K. Rowling