I once pulled a pratical joke in class.
Requirements:
Can of Oatmeat (One you can open by hand)
Bottled Water (If heated, even better)
Fart Bomb (The kind with the smaller packet that you pop to start)
Procedure:
While no one is looking...
Open oatmeal, mix with water
Place fartbomb on desk
Start fartbomb
Go "bleeeaaah!!!" while throwing wet oatmeal on fart bomb
Get up and run to the trashcan (aka get out of the danger zone)
Watch the "vomit" explode and smell awful