This old classic is one of the 2M pranks favorite!
What you need for this one is:
A funnel
Glass of cold water
Coin
First you attach the funnel in your belt buckle so it faces the inside of your pants. Next you lean your head backwards and you put a coin on a top of you forehead. Now you try to hit a funnel with a coin. The outcome is not important. Next you challenge some one to do the same thing. Once he/she is in a position you take a glass of water and pour it in the funnel.
