Why does sucking on helium make your voice sound funny?

If you're into that old party trick, I can tell you how it works. Though why you want to sound like a munchkin, I don't know.

Helium is a colorless and odorless gas and, as we know, lighter than air. It changes the voice by altering the environment in the place where sounds are formed—the throat

The vibration of the vocal chords in the throat is what causes sounds. What happens when you inhale helium is that the nature of the air in the throat is changed.

It's now much lighter than usual. And that causes our vocal chords to vibrate much faster, leading to a higher voice.

So now you know how and why it works. But keep in mind that sucking helium is not a healthy pastime. And never inhale helium from a pressurized tank.