You can breathe in helium and sulfur hexafluoride
to change the pitch of your voice and illustrate how density affects the speed of sound. Helium is easy to come by... you can pick up a helium-filled balloon at many grocery stores. To make your voice higher, you just exhale air, take a deep breath of helium and talk (or sing, if you're extroverted). While helium is about six times lighter than air, sulfur hexafluoride is about six times more dense. It isn't as easy to find as helium, but you may be able to get some from a specialty gas store that carries oxygen, argon, etc. You can make yourself sound like Barry White as opposed to Alvin of the Chipmunks by doing the exact same thing as you did with the helium.
Neither helium nor sulfur hexafluoride
is toxic, but both can make you lightheaded from breathing them instead of air with oxygen, so use common sense. Don't keep breathing either gas. Exhale after breathing them, then take a deep breath of regular air. Incidentally, it is a myth that you have to bend over to expel the dense sulfur hexafluoride gas. Your lungs are more than capable of breathing it out without any exotic posturing. Have fun, but be safe!Rainbow in a Glass Density Column
| Elements Listed by DensityPhoto: Space-filling model of sulfur hexafluoride (Ben Mills)