beer pong
Beer Pong is a game played by 4 players, two on each side.  It is played with two ping pong balls.  Six cups are then placed on each side in a pyramid fashion (six on each side).  Also to keep the game sanitary, a cup of water is placed on each side to rinse the ball in case it drops on the floor.  The cups are filled half full (roughly 2 beer cans will be used for each side).  
The object of the game is to shoot the ping pong balls into the opponents cups, making them drink.  Once all cups on one side of the table are made, then the team who made the cups win.  
Special rules:  Make it till you miss... when a team makes the last cup, the opposing team has an opportunity to make all the remaining cups as long as they do not miss, if they do, game over.
Satellites - the cup you are drinking out of is your satellite, if an opposing player makes it into your cup, game over.
Bounces - bounces can be off of anything, the table, wall, a person, as long as it bounces, a bounce counts as two cups.
If both players from one team makes the same cup in the same turn, they win.
Every house has different house rules, usually posted on the wall.  Find out what house rules they use when playing and abide by them.
Started trippin on mushrooms while playing beer pong and the cups seemed to be moving around... and i still went 15 games straight