If you breathe in helium and talk, you will have a squeaky (but not higher) voice. Learn how to do the helium voice experiment safety and find out how helium voice works.
How to Get Helium Voice
You can breathe in helium to change the sound of your voice and illustrate how density affects the speed of sound.
You can pick up a helium-filled balloon at many grocery or party supply stores. To make your voice squeaky, you just exhale air, take a deep breath of helium and talk (or sing, if you're extroverted.)
How Helium Voice Works
When your vocal cords vibrate, as when you talk or sing, the sound waves are propagated through helium instead of air.
Helium is about six times lighter than air, so the sound waves travel much more quickly through helium than through air. While the geometry of your vocal cords doesn't change, they vibrate differently in the lighter gas.
The actual pitch of your voice does not change very much. However, the resonances associated with your voice are present in different proportions.
Helium Voice Safety
Helium is non-toxic, but this project can make you lightheaded from breathing in helium instead of air with oxygen.
Don't breathe in more than a few breaths of helium. Exhale fully after each breath, then take a deep breath of regular air.
Don't repeat the helium voice project over and over again. Never breathe helium directly from a compressed gas canister.