So does soapy water. Penn and Teller suggest this trick next time you’re travelling on business or with a friend who’s staying in a different hotel room. While in his/her room, use the bathroom, wet your finger and very lightly touch the bar of soap. Then right “Nice buns” or some such on the mirror. If you do it right, it won’t be visible in normal conditions.
But the next time your friend takes a shower, the mirror will be fogged, except where you wrote. It will look as though someone came in and wrote “Nice buns” while he/she was showering! Spooky.
I think this is in Cruel Tricks for Dear Friends.