The prefects' bathroom

The prefects bathroom is mentioned in The Goblet Of Fire, when Cedric Diggory helps Harry with his golden egg's clue. Harry had already helped Cedric by telling him what the first task was, so Cedric pays him back. The prefects bathroom consists of a swimming pool sized bath, and a mermaid. Moaning Myrtle tells Harry that Cedric opened the egg underwater, so Harry does the same. The merpeople in the egg sing to harry, telling him exactly what he has to do in the sencond task: to retrieve something he loves from the botom of the black lake. This item happens to be Ronald Weasley.