Professor Snape: Potter! What's your hurry?
[Harry walks up to Snape]
Professor Snape: Congratulations. Your performance in the Black Lake was inspiring. Gillyweed. Am I correct?
Harry: Yes, sir.
Professor Snape: Ingenious.
[Climbs up ladder in his personal store room]
Professor Snape: A rather rare herb, gillyweed. Not something found in your everyday garden.
Professor Snape: [Picks up small vial, then climbs down to floor. He then shows the vial to Harry] Nor is this.
Professor Snape: Know what it is?
Harry: Bubble juice, sir?
Professor Snape: Veritaserum. Three drops of this and You-Know-Who himself would spill his darkest secrets. The use of it on a student is, regrettably, forbidden. However, should you eve steal from my personal stores again, my hand might just slip...
[turns vial sideways, then upright again]
Professor Snape: ...over your morning pumpkin juice.
Harry: I haven't stolen anything.
Professor Snape: Don't... lie... to me! Gillyweed may be innocuous, but boomslang skin? Lacewing flies? You and your little friends are brewing Polyjuice Potion, and believe me, I'm going to find out why!
[Slams door shut in Harry's face]