Hermione Granger: You've done it, Neville! You've found the Room of Requirement!
Ron Weasley: The what?
Hermione Granger: It's also known as the Come and Go Room. The Room of Requirement only appears when a person has real need of it, and is always equipped with the seeker's needs.
Ron Weasley: So, say you really needed a toilet...
Hermione Granger: Charming, Ronald. But yes, that is the general idea.
Harry Potter: It's brilliant! It's like Hogwarts wants us to fight back!