Solid, liquid, or gas? Many things are solids at room temperature and normal air pressure -- like a chunk of iron, or a stick of butter. Others are liquid -- like water, or olive oil. And still others are gases--like oxygen or nitrogen. The universe is full of substances that behave differently at normal atmospheric pressure and temperature here on earth.

Fast fact: While we depend on oxygen for our very life, oxygen isn't the main gas whizzing around in the air. Earth's air, in fact, is 78 percent nitrogen gas.

The "boiling point" is the temperature at which a substance turns from liquid to gas. Water's boiling point is 212 degrees F. We can turn water to gas -- steam -- by putting a pan of it on the stove to heat. Eventually, the water will begin to bubble, and we'll see steam escaping into the air. The water has changed from one state -- liquid -- to another: gas.

Nitrogen's boiling point must be very different than water's, if nitrogen is already in the "gas" state when the temperature is, say, 72 degrees F. More amazingly, nitrogen is still a gas when it's 10 degrees outdoors -- or 10 below zero. Nitrogen, after all, doesn't condense into liquid and begin dripping onto our cars in winter. So its boiling point must be lower still.

What IS the boiling point of nitrogen? At ordinary pressures, nitrogen begins to boil at an astonishing -320 degrees F. Anything above -320, and nitrogen's a free-wheeling gas. Since the coldest it naturally gets on Earth is about -128 degrees F, we're in no danger of our air going liquid on us.

But below -320 F, nitrogen does -- finally -- condense into a colorless, fragrance-free liquid. Lower the temperature just a bit more, to -346 F, and nitrogen obediently freezes into solid ice.

Liquid nitrogen is made by squeezing air through a compressor, cooling it, and then distilling out the nitrogen from other liquefied gases, like oxygen and carbon dioxide. Open a container of frigid liquid nitrogen and it will begin furiously boiling, the nitrogen turning to gas and vanishing into the air.

Some liquid nitrogen tricks: An inflated balloon plunged into liquid nitrogen will collapse, re-inflating when pulled out. A marshmallow dipped in liquid nitrogen, then hit with a hammer, will shatter like glass.

Liquid nitrogen is used to instantly flash-freeze food, preserve stored blood and organs, in surgery to deep-freeze skin cancers, and in movies to create billowing fog. Liquid nitrogen can even be used to make ice cream (but don't try this at home!). To learn how, visit the website www.polsci.wvu.edu/Henry/Icecream/Icecream.html .

Here on Earth, we need machinery to coax nitrogen out of its gas state. But on the planet Pluto, where temperatures hover around -380 F, the ground is decorated with nitrogen frost. And on Neptune's biggest moon, Triton, icy volcanoes may spew liquid nitrogen like lava.