Cool tricks with relative density by Adam Weiner

Is it magic? Is that aluminum foil boat floating on air? Well, no and no. What we literally don't see is that the bottom of that aquarium is filled with sulfur hexafluoride (SF6). Sulfur hexafluoride is a relatively nonreactive gas that has a density of about five times that of air. It's also transparent.

So, if you make a boat with a light enough hull - out of aluminum foil for example - although its density is going to be greater than air, it just might be less than SF6, and it's going to float there all day given the chance. (Remember, for a boat the density is the total mass of the hull, plus the air space within, divided by the total volume.) To sink the foil boat all you have to do is bail some of the gas into the boat, increasing its density until it submerges - just like with a boat in water.

Another property of sulfur hexafluoride was demonstrated on an episode of Mythbusters a while back. Because sound travels more slowly in SF6 than it does in air, if you breathe it into the lungs, it causes the vocal cords to vibrate at a lower frequency than they do in air, resulting in an abnormally deep voice. It's well known that helium, which is less dense than air, produces the opposite effect - that comically high-pitched voice (sometimes demonstrated at parties) if you inhale it and speak. Be aware that inhaling either of these gases displaces oxygen from the lungs, and done to excess can be dangerous.

Nevertheless, with a few liters of sulfur hexafluoride, you can make your own aluminum foil regatta too. Whatever floats your boat!
Source: http://www.popsci.com/scitech/article/2008-12/whatever-floats-your-boat