I used to work in printing factory years ago. Well some guys had to wear respirators to work in specific areas. A friend of mine put his respirator down to go get something out of the vending machines. I took it and put blue ink all around the inside of it and put it back where he left it. He returns to the printing press puts back on the respirator. No less than two minutes later he's paged to the supervisors office......no I didnt tell him he had a blue ring around his nose and mouth. He comes back five minutes later madder than hell at me. Of course there was ten other guys waiting for him all laughing their asses off.