 The crash lands them in hot water with headmaster Albus Dumbledore and Minerva McGonagall , Head of Gryffindor House and the Transfiguration professor , who doesn ’ t expel them in spite of the clear breach of magical law but does assign them to detentions : Ron must clean the school trophies in the trophy room , and Harry must help the lunatic Professor Lockhart with addressing his fan mail . Lockhart ’ s classes turn out to be as obsessed with the man as he is with himself , as proven by a “ hands @-@ on ” training experience with a pack of destructive pixies ( small , bluish creatures that exist to cause mayhem ) , and Harry learns that prejudice exists in the wizarding world by the way of blood status : Those with “ pure ” blood ( only wizarding heritage ) have the advantage over and condescend to those with Muggle parentage . Harry also begins hearing an unnerving voice ( one only he can hear ) seemingly coming from the walls of the school itself , and during a “ death @-@ day party ” for the Gryffindor ghost , the trio happens upon a petrified Mrs. Norris , the cat belonging to school caretaker Argus Filch , and a warning scrawled across one of the walls : “ The chamber has been opened . Enemies of the heir , beware . ” 
