MythBusters2003

Himself - Host:
I promise to show you something really cool, as long as you promise *not* to try it at home. Okay? Okay. Now, everyone wants to know why my voice sound higher when I inhale helium.

[inhales from balloon filled with helium]

Himself - Host:
[with high-pitched voice] The simple fact is that helium is six times less dense than air, which means sound waves travel through it much faster, which makes my voice sound much higher. Now, the same effect can be achieved in reverse if I inhale something like sulfur hexafluoride, which is six times *denser* than air. I inhale some of that...

[inhales sulfur hexafluoride from another balloon]

Himself - Host:
[now speaking in a deep satanic voice] And my voice gets really low, although somehow I'm still funny. It's scientific!