I used to have some people in CS class I used to play dirty tricks on to irritate them.
Our school network had a folder where scripts in a certain format were called. I could use these scripts to determine who logged in (based on student ID) then send them a basic message and log them out. I could also get them in trouble by using a VPN and a tunneler to put some porn up on the screen.
Put mentos in ice cubes and then hand your friend/target a glass of coke with said ice cubes :P
1. Fill ice tray with each slot just under half full, and wait for it to freeze
2. Place mentos on top of each frozen half-sized cube
3. Fill the rest of each cube slot full and freeze, completely encapsulating ice cubes
5. Provide free refills until it's time to hide
@Cheraphy: why would you tap an airhorn? Also, several services and processes restart explorer.exe
you tell your friend that theres this awsome drinking game, you crack open a beer each and you explain that you have to balance a grape or buiscut or whatever on your head and you have to drop it into a funnel thats stuffed in your belt, you get your friend to balance thing on his head and then you pour your beer down the funnel