Everyone knows about the old trick where you inhale helium and it makes your voice sound really high pitched. The reason is because helium is much less dense than air, and it makes sounds travel through it faster, increasing the pitch.
But what many people don't know is that the general principal works just as well in reverse, as MythBusters star Adam Savage demonstrates:
I've always wondered what kind of gas I could use to do that. Apparently xenon also works, but it has an anaesthetic effect so it probably wouldn't be a good choice. So it seems SF6 works well.
Apparently its expensive, but I must find some!