Lather, rinse, and repeat

Disclaimer: You know it. . .

Harry, Hermione, and Ron are standing in the common room at two in the morning, whispering about something.

Hermione: We could get in trouble for doing this, maybe even get expelled. We're in deep water as it is.

Harry: I know, but it'll be worth it.

Ron: When are we going to do this?

Harry: During class, in front of everyone. Then, they walk up their respective staircases and go to bed.

The next day during Defense against the Dark Arts. . .

Harry (whispering to Ron and Hermione): Did you bring your supplies?

Ron and Hermione (hold up their supplies)

Harry (rubbing his hands together and grinning evilly): Excellent

Snape: All right class, today we're going to learn about-

Harry: NOW! (runs up behind Snape and pulls his arms behind him)

Snape (trying to break free): POTTER! WHAT ARE YOU DOING?!

Ron (pours water on Snape's head)

Snape (spitting water): WHAT?!

Hermione (squirts shampoo on his head and scrubs it)

Ron (pours more water on Snape's head): There! All clean!

Everyone (except for the Slytherins): YEAH!

Harry, Ron, and Hermione: Revenge is sweet! HAHAHAHA!

Snape: You three are in so much trouble. . .