If you watch 'Live With Kelly'  then you may have seen her and co-host Neil Patrick Harris as they talked with Science Bob recently about Sulfur Hexafluoride. 

How come I've never heard of this before?  If helium makes your voice go high because it is 6 times lighter than air, and  sulfur hexaflouride is 6 times heavier would it make your voice go low?  Don't Try This At Home (not that most people have sulfur hexaflouride in their house but it seemed like the right thing to say)!