Harry starts classes at Hogwarts School, with lessons including Transfiguration with Head of Gryffindor, Minerva McGonagall, Herbology with Head of Hufflepuff, Pomona Sprout, Charms with Head of Ravenclaw Filius Flitwick, and Defence Against the Dark Arts with Quirinus Quirrell. Harry's least favourite class is Potions, taught by Severus Snape, the vindictive Head of Slytherin who seems to loathe Harry. Harry, Ron, and Hermione become far more interested by extracurricular matters within and outside of the school, particularly after they discover that a huge three-headed dog is standing guard over a trapdoor in a forbidden corridor.