Helium's atomic number is 2 and its atomic weight is 4.002602. Its boiling and melting points— -452.1°F and -458.0°F, respectively—are the lowest among the elements. It is the second most abundant element in the known universe (after hydrogen). And it makes your voice sound really funny when you inhale it. Here's why.

A Crash Course in Sound

When you speak, air travels up from your lungs and through the larynx, where it meets the vocal cords (or vocal folds), twin infoldings of mucous membrane stretched horizontally across the larynx, and hits the underside, causing them to vibrate. The vibration of the cords excites air molecules in your vocal tract and sets up resonant frequencies. The vibration of the vocal cords influences the pitch (the perceived fundamental frequency of a sound) of your voice; the vibration of the air in the vocal tract influences your voice's timbre (the quality of a sound that distinguishes different types of sound production—remember that for later, it will be important); and manipulation of the vocal tract—moving your tongue, lips, etc.—creates different resonant frequencies and allows you to make the different sounds of speech, like "oohs" and "aahs." Your voice finally leaves your mouth in the form of waves, oscillations of pressure transmitted through a medium.

Voice, Meet Helium

In addition to the vibrations and manipulations that influence the sound of your voice, what another person hears when you speak also depends in part on what the space where the sound is created contains. The air that fills a room where you might be speaking to someone is made up of roughly 78.08 percent nitrogen, 20.95 percent oxygen, 0.93 percent argon, 0.038 percent carbon dioxide, and tiny amounts of other gases. Nitrogen, which makes up the majority of our air, has a mass roughly seven times greater than that of helium. Because helium is lighter than air, sound waves travel through it faster. In a room where the temperature is 68 degrees Fahrenheit, sound travels at 344 meters a second through air, but 927 meters a second through helium. When you inhale helium, you're changing the type of gas molecules in your vocal tract and increasing the speed of the sound of your voice.

Some people think that helium changes the pitch of your voice, but the vibration frequency of the vocal cords doesn't change along with the type of gas molecules that surround them. When your vocal tract is filled with helium, your vocal cords are vibrating at the same frequency as usual. It's actually the timbre (again, the quality of a sound that distinguishes different types of sound, also known as tone quality or tone color) that changes, because those lighter-than-air helium molecules allow sound to travel faster and change the resonances of your vocal tract by making it more responsive to high-frequency sounds and less responsive to lower ones. Your voice winds up flat and Donald Duck-esque and listeners perceive this as a change in pitch.

More Fun With Gases

If a lighter gas like helium gives us a squeaky-sounding voice, you might to assume that a heavier-than-air gas would amplify the lower resonant frequencies and make it deeper and richer (trading Donald Duck for Barry White, if you will). You'd be correct; gases like xenon and sulfur hexafluoride slow the speed of sound and lower the resonant frequencies of your vocal tract. 

No Fun With Gas

As amusing as the results are, inhaling helium is not so great for you. While you're inhaling it, you're not getting the oxygen you need for normal respiration. Breathing helium continuously can cause asphyxiation within a few minutes. That light-headed feeling you get from a few inhalations is a sign you need to take a break. And please don't ever inhale helium directly from one of those pressurized tanks. The high flow rate can rupture your lung tissue or send a concentrated mass of gas into your bloodstream, after which it can lodge in the brain and cause a stroke, seizures and death.

This story originally appeared in 2009.