Science & TechFact
Helium doesn't raise your pitch. Seriously.
With helium in your lungs, your vocal cords vibrate at exactly the same rate, so the pitch stays put. But sound travels almost three times faster in helium than in air (about 970 versus 343 m/s). That shifts the resonances of your throat upward, boosting the high overtones, and your voice sounds squeaky like a cartoon character.
Useful: Sulfur hexafluoride does the opposite and makes you sound like a movie villain; both gases displace oxygen, so be careful.
Source: en.wikipedia.org