The Weasley twins set off one last prank as a decoy so that Harry can talk to Sirius via the fireplace in Umbridge's office - the only one not under surveillance - and are caught; they decide to leave Hogwarts in spectacular fashion, leaving a magical swamp inside the school, and to use the winnings from the Triwizard Tournament given to them by Harry to start their joke shop,Weasley's Wizard Wheezes.