Inhaling helium

For Halloween, a friend had a helium balloon that he was playing with.  He would inhale a little bit of helium and then talk in a high, squeaky voice.  I wondered why this happens and researched it.  Helium is less dense than air.  Helium's speed of sound is also 3 times faster than that of air, meaning that helium's wavelengths have a higher frequency.  Because of these two factors, the speed of sound in the voice is increased when helium is inhaled. This increases in wavelength increases the pitch of the resonating frequencies of the vocal tract.

One could lower the pitch of the voice by inhaling a gas that is more dense than air.  Denser gases include sulfur hexafluoride and xenon.