Question: why does helium make your voice high pitched

Jennifer Rudd answered on 9 Mar 2015:

I had to look up the answer to that! This is what wikipedia says: The speed of sound in helium is nearly three times the speed of sound in air. Because the fundamental frequency of a gas-filled cavity is proportional to the speed of sound in the gas, when helium is inhaled there is a corresponding increase in the resonant frequencies of the vocal tract.

Basically, because helium travels faster than air it changes our vocal chords and makes them tighter, which makes our voice higher!