This might seem like a magic trick, but it's actually simple science: Sulfur hexafluoride, a gas that's six times heavier than air, is placed in a covered tank to ensure that any random breezes don’t come along and sweep it away. Once you take the cover off the tank, an aluminum boat or something similar will float on the layers created by the heavy gas below and the normal oxygen above, creating a seemingly gravity-defying piece of aluminum foil.
Sulfur hexafluoride isn't just useful for cool science experiments. It also powers torpedo systems, is combined with air to fill tennis balls, and is used in retinal detachment surgeries (it was also used in the air bags of certain Nike shoes between 1992 and 2006). And because it's so much heavier than helium, it has the opposite effect on your voice when you inhale it, as you can hear in this hilarious clip of Kelly Ripa and Neil Patrick Harris trying it out.