He turned his wand on Harry so fast that Harry reacted instinctively; all thought of nonverbal spells forgotten, he yelled, “Protego!”
His Shield Charm was so strong Snape was knocked off-balance and hit a desk. The whole class had looked around and now watched as Snape righted himself, scowling.
“Do you remember me telling you we are practicing nonverbal 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. Several people gasped, including Hermione. Behind Snape, however, Ron, Dean, and Seamus grinned appreciatively.