I sh!t on myself

Sulfur hexafluoride is an interesting gas, it basically has the oposite effect of helium. It is heavier than air (over five times denser) and when inhaled it makes your voice deeper, talk in about 3 octaves lower.

You can buy a compressed cylinder from sigmaaldrich.com , though I must say $229 seems a trifle expensive for a joke.

The tank is filled with sulpher hexafluride which will stay at the bottom acting as if it were a solid and keep anything lighte enough up, making it look like it is floating.

View also Mercury Thiocyanide.