VIDEOS:
Kripke's Helium Prank
While Sheldon is on the radio with NPR, Kripke fills his office with helium, causing his voice to become increasing squeaky. Even worse, Kripke has the entire cafeteria, including Sheldon's friends, hear the broadcast.
Example of:
Helium Speech
"PJ" (Lilo & Stitch)
Example of:
Practical Joke