Tell someone you can pin a glass of water to the wall -- a real glass, not a paper cup, using an ordinary straight pin. Naturally they won't believe, so you set out to prove it.
Get a glass of water and a pin. Hold the glass up to the wall and start to pin it up. And then drop the pin. You've got the glass in position just right, so you ask your victim real nice to get the pin for you. When they bend down to pick it up, dump the water on their head.
This works especially well when there's a crowd of people watching. It can also be very dangerous for the joker, so be careful if you try it.