Why does helium change the pitch of your voice?

Helium is a very light gas. Light gases tend to float (as seen in a helium filled balloon). It is a light gas because it moves very very fast. It is said to be NOT dense.

When you have sucked Helium in, your breath that you use to speak will pass through the Helium gas. Your breath will now be moving faster. Things that move faster tend to have higher pitch.

Note: Helium does not change your vocal chords or your vocal muscles... just the speed of your breath.

Also, Argon gas can make your voice sound lower. But this is dangerous because it can sink into your lungs. So I was told you should be hanging upside down if you ever want to take a sip of Argon gas. I have never done this. DO NOT ATTEMPT this. You could pass out and suffocate if no one is around you.