- "Snape had given them so much homework, Harry thought he was likely to be in the sixth year before he finished it. Ron was just saying he wished he had asked Hermione how many rat tails you were supposed to add to a Hair-Raising Potion when an angry outburst from the floor above reached their ears."
The Hair-Raising Potion is a potion that causes the drinker's hair to stand on end. One of the known ingredients is rat tails. Harry Potter and Ron Weasley had to complete a homework assignment which involved this potion in their second year.
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