Do you remember me telling you we are practicing non-verbal spells, Potter

Oscar-winning composer AR Rahman sings at the opening ceremony

"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."
— J.K. Rowling (Harry Potter and the Half-Blood Prince (Harry Potter)