Here's an original pour some water into your hands and act like you making a sneeze sound behind one of your friends
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do something funny
pull a prank on him
load their drink with salt (works best with soda)
You can view videos of the skirt pull prank and learn how to perform it from the videos at the YouTube website. Once on the website, type "Skirt pull prank" into the search field at the top of the page and press enter to bring up the information.
they drew on his face
Peter put a dead mouse on Mr. Dussel's bed as a prank.
A fart machine in there desk
Give someone a swirly or wedgie
Put gum in his hair
The best prank apps for iPod Touch 4G are Pallaudio, Scary Prank, and Ghost Camera.
That depends if you want to get into trouble or not.