Great trick if your most hated teacher is called Mr Boyle, who is a fat, sweaty, balding german teacher with no sense of humour, like ours was. Set up a kettle in your room (oh, it has to be a boarding school). Upon seeing Mr. Boyle approaching, switch the kettle on, and lean out of the window, yelling 'Boyle, you bastard!' Then duck out of the way before the fat fucker could see you. When Boyle came waddling in, red faced, to investigate, the shouting would continue in such a way that when Boyle entered the room it would appear you were cursing the kettle for its poor performance. Whoever came up with this, I salute you.