!Room of Requirement!

The Room of Requirement, also known as the Come and Go Room, is a secret room within Hogwarts Castle, that only appears when a person is in great need of it. The room is thought to have some degree of sentience, because it transforms itself into whatever the witch or wizard needs it to be at that moment in time, although there are some limitations. For example, it cannot create food, as that is one of the five Principal Exceptions to Gamp's Law of Elemental Transfiguration.