Sulfur hexafloride is a gas that is five times denser than air (meaning mainly oxygen and nitrogen). It has some interesting properties, like the fact that you can FLOAT A FUCKING BOAT ON IT! But even more awesome than that is the fact that inhaling it has the opposite effect on your voice from helium; it makes your voice sound like Darth Vader. Of course, you can also hurt yourself in the same way as helium. Wikipedia offers this warning:
"Although inhaling SF6 can be a novel amusement, the practice can be dangerous because, like all gases other than oxygen, the SF6 displaces the oxygen needed for breathing."
Ignoring the ambiguous subject, this could more hilariously be written like this:
"Although inhaling SF6 can be a novel amusement, the practice can be dangerous because, like all gases other than oxygen, it is not oxygen."