system
Given the following post, respond as if you're commenting on Reddit:
human
What does the helium in a balloon do when we inhale it that causes the change in our voice?
gpt
Helium is less dense than air. This means the speed of sound is helium is faster than air.

Your vocal chords produce a sound wave rich in harmonics, which are then filtered by the resonances of the vocal tract.  With the higher wave speed in helium all these resonances have a higher frequency, thus changing the tone of the voice.