yes - i had fun with a surveillance team who put a fluorescent dye
into the water supply at the hotel i was staying at.  i'd noticed that
it was slightly brighter than usual water: it glowed in sunlight.
after washing, i rubbed my skin - hard - placing a circle encompassing
my forehead, cheeks and neck, then a straight line right down across
my nose, and another two lines across my cheeks.  you might recognise
this as the CND "Ban The Bomb" symbol.  the smirks on the faces of the
surveillance team - identifiable as the ones with polarised glasses -
were worth it.