The windows in some of our classrooms would often steam up with condensation during lessons.
We discovered that if you had greasy skin (as many teenagers do), you could rub your fingers on your face, transferring some 'facial grease' onto them and then write 'invisible' words on the glass when dry, e.g "Bill Marlow is a Cunt".
When the windows next steamed up (which could be during another class) the invisible words would miraculously 'appear'. Often the blame would be directed at innocent pupils and once a whole class got detention for not revealing who had done it; they really didn't know! IT WAS ME! HA!