Take a sheet of cardboard or a throw away magazine, form a cone with it.
Take the cone, a coin, and a liquid refreshment (water causes least damage)
in a bottle or a cup, of course you will be pretending its your drink.
Challenge the victim (bet a sum), that they can not drop the coin, placed
on their forehead, with their eyes closed, into the top of the cone shoved
into their pants at the waist within so many tries.
To prove that it is possible, demonstrate the procedure a few times, you'll
be supprised that it is possible. (practice before hand)
When the victim tries it, as soon as the eyes close, pour the liquid down
the cone.