You've seen people suck up some helium and talk like a chipmunk at birthday parties, but now you can hear the opposite. This selfless science teacher inhales sulfur hexafluoride and treats us to the Oscar Mayer song.
I've already told you why helium makes someone's voice sound high and squeaky. Different gases change the kinds of resonant frequencies inside a person's throat. Helium doesn't interact with much, and it's very light. That means that it won't glom on to molecules inside your body, and it will be easily pushed out by the lungs. The only danger is making sure you don't inhale too much and suffocate yourself.
Sulfur hexafluoride isn't as good as helium. It is chock full of extra electrons, just looking for somewhere to be dumped. It's also a great deal heavier than air. This means that it isn't as easily vacated by the lungs. Even a short breath or two can pool gas in the lungs. (If you see other demonstrations of this, it's not unusual for the demonstrator to double over at the end, trying to get the gas to pour out of their lungs.) However, sulfur hexafluoride is technically inhalable, and it makes you sound exactly like a record on slow speed. Check it out in the video.