How can you make water invisible?

Is invisible water for real? Well, yes and no. Swissmediavision/Getty Images

The magician's silk-gloved hands dance through the air, delicately pulling back a sheet to reveal the prop for his next trick: a glass fish tank. It appears empty, and yet the illusionist insists that the aquarium contains a most remarkable substance: invisible water. To demonstrate, he places a tinfoil boat inside the tank, and it merrily bobs up and down, as if afloat on some unseen surface.

Next, the magician produces a plastic cup and begins bailing out the tank. It appears as if he's doing little more than miming, and yet the unseen water level seems to lower with each cupful. The little boat even bobs more and more, as if each dip of the magician's hand disturbs the surface more.

Perhaps you've seen this feat performed on a talk show or in Internet videos. How's it done? The practitioner isn't using sleight of hand, wires or digital special effects to float the tiny boat. The "magic" at work is none other than an invisible gas called sulfur hexafluoride (SF6). This curious vapor behaves like phantom water because it's heavier and denser than air. When poured into a fish tank, it sinks to the bottom and can support the weight of very light objects floating on its surface.

Sulfur hexafluoride is completely nontoxic and nonflammable, making it ideal for various scientific uses. Medical professionals use it as a test gas when studying­ the respiratory system, and the electric power industry widely uses it as insulation.

The substance has shown up in everything from Nike basketball sneakers to NASA spacecraft, but the in vis­bile water illusion certainly isn't rocket science. Read the next page to learn what makes the illusion possible.