It’s Fun to Know: Why Inhaling Helium Gives You a Squeaky Voice
by ETR (05/6/2008)
Helium is one-seventh as dense as the air we breathe. When you inhale the gas and then exhale, the helium escaping from your lungs causes your vocal cords to vibrate much faster than normal. The result is a high-pitched, squeaky voice.
(Source: Wired)