Severus Snape

A Potions classroom in the dungeon. Dim candlelight reflects off glass vials containing ingredients. Severus Snape slowly walks down the rows, stopping at a student's desk.
Severus Snape: Let's see. Is that little bubble rising above your cauldron like a balloon again? Tell me, weren't you warned about the dangers of overheating?

• I thought...
  "Thought?! That's the trouble... Sometimes it's better not to think at all. Let this serve as a lesson: in potion-making, there's only one rule—follow the recipe. So, immediately reduce the flame and stir the mixture exactly seven times counterclockwise."