Saying that a substance is “heavier than air” doesn’t sound too exciting, until you see the borderline terrifying shit sulfur hexafluoride can do. For instance, most of us have, at some point, huffed on a helium balloon so that we can pretend we’re the Chipmunks and make them say swear words. But helium isn’t the only substance that will mess with your vocal range. Sulfur hexafluoride is helium’s evil twin, like Bizarro is to Superman, or Lady Gaga is to Madonna. Huff a balloon full of this stuff and your voice will take on an ominous baritone, as demonstrated hilariously by Kelly Ripa and Neil Patrick Harris.
This happens precisely because sulphur hexafluoride is several times heavier than regular air, so it lowers the frequency of any sound passing through it, which is exactly how helium works, except, you know, in reverse. But once you know that the stuff is heavier than air, you can start to do really cool things with it. For instance, if poured into a glass, sulfur hexafluoride sinks through normal air and pools invisibly at the bottom of the container. Just like water, it can support the weight of less dense objects on top of it.