He, he, helium.

After Thanksgiving dinner, you might resemble a blimp, but you probably won't feel lighter than air. Why? The answer is in the stuffing: a heavy holiday meal versus a lightweight gas called helium.

Helium is the stuff that keeps balloon characters like Bart Simpson (left) flying high in the Macy's Thanksgiving Day parade. In fact, there's enough helium inside Bart to fill more than 12,000 party balloons (and you thought Bart was just an airhead). Read on to find out more hot stuff about helium.

What makes helium-filled balloons or blimps float?

Helium (He), the number-two element on the periodic table, is lighter than air. Elements, the different kinds of atoms from which all materials are made, are numbered from lightest (hydrogen) to heaviest (number 109, which has not yet been named). Since air is mostly nitrogen and oxygen (numbers seven and eight on the table), helium gas weighs less than air. That also means helium is less dense--has less mass in a given volume--than air. So helium-filled airships float up into the wild blue yonder.

How can helium lift something as heavy as a blimp carrying passengers?

A blimp--a huge floating balloon "envelope" with a cabin, engines, landing gear, and passengers--can weigh up to 7,200 kilograms. But because helium is so lightweight, it has lots of lifting power, or upthrust. Inflating the air-ship's envelope with the right amount of helium--about 7,080 cubic meters' worth--can overcome the downward pull of gravity. The upward lift gets the airship off the ground for smooth floating ahead!

What goes up must come down.... But how?

Inside the envelope of a blimp are two collapsible airsacs--one at each end. Fans blow "heavy" air into these sacs. The air acts as weight, or ballast, like the sandbags of old-time, hot-air balloons. When the airsacs are empty, the blimp flies high; when filled, the airship sinks to the ground.

Can you explain the difference between a helium balloon or blimp and a

hot-air balloon?

Hot-air balloons work the same way helium fliers do because the hot air inside the balloon is less dense than the air outside. Here's how these balloons work: Two giant burners inside the balloon heat the air. The added heat energy makes the air molecules jump around. The movement pushes the molecules farther apart, expanding the balloon. This less-dense air lifts the balloon off the ground. Hot-air-balloon pilots must constantly reheat the air to keep the balloon up. When it's time to land, they just let the air cool.

Didn't a famous airship--the Hindenburg--once burst into flames?

The Hindenburg, a German passenger airship, became famous when it burst into flames while docking in New Jersey in 1937, after a transatlantic flight. The Hindenburg, like many airships built during the early 1900s, used a hydrogen-filled envelope, not helium. Hydrogen (H) is lighter and was easier to get.

Hydrogen, however, is extremely flammable. Nobody knows what sparked the Hindenburg fire, but flames torched the airship, killing 35 people. That's why today's blimps use helium, a non-reactive, or inert, noble gas.

What's so "noble" about helium?

That's the name given to the six elements in the far right column of the periodic table. Atoms of these elements have a full set of electrons in their highest energy "shell." Having "extra" electrons or electron "vacancies" to fill makes some elements chemically reactive. Helium has neither, so it doesn't participate in chemical reactions.

Where is helium "made"?

Hint: Helium's name comes from helios, the Greek word for sun. Stars, including the sun, have tons of hydrogen atoms crashing together under intense heat and pressure. The collisions fuse the hydrogen nuclei, or centers, to form heavier elements, including helium. The fusion reactions produce enormous amounts of energy in the forms of heat and light.

So where can you find helium on Earth?

Helium, one of the 92 naturally occurring elements in the universe, is odorless and colorless--which makes it difficult to find. But scientists have found the gas mingled with underground natural gas deposits trapped in porous rock formations, called dolomite, in Texas, Oklahoma, Wyoming, and Kansas. In fact, 90 per cent of Earth's helium is mined in the United States. Dr. William D. Leachman of the Bureau of Mines says the U.S. has mined and stored enough helium to last us about 100 years.

Where else can we spot helium in action?

Besides its use in keeping blimps and party and parade balloons aloft, helium is also used by the military in airships used for surveillance and by NASA for weather experiments.

And keep looking skyward. This year, five teams of long-distance balloonists will be racing helium airships around the globe.

What a gas!