This is a very very funny prank and very very simple. P.S. Please vote for me in the April fools day contest.
Step 1: Things You Need.
1x Bottle of water
1x Friend with a good sense of humor
Step 2: The Funnel.
You tell your friend to stick the funnel in his pants.
Step 3: The Quarter.
Next you tell you friend to balance the quarter on his forehead and tell him to try to make it fall in the funnel without using his hands.
Step 4: The Water Bottle.
While he is looking up and balancing the quarter on his forehead, pour the content of the bottle down the funnel. And that , my friends, is how you do the water down the pants prank.