How many of us have taken a breadth from a helium balloon? It's always good for a laugh, but it is also an opportunity for some good physics.
One of the most important aspects of sound that determines what you perceive is the sound's frequency - how many times per second the air pressure changes. Your vocal folds (or cords) determine the frequency of the sound you produce. Breathing in helium doesn't change this.
What the helium does change is the timbre of your voice.
Two musical interments can play the exact same pitch, but no one will ever confuse a flute with a trumpet. One of the factors that determine timbre is the harmonic structure of the sound being produced. For example, when a piano wire vibrates 440 times every second (440 Hertz), it is also vibrating at 880 HZ, 1320 Hz, 1760 Hz, etc. However, which harmonic frequencies are produced is partly determined by the speed of sound in the vibrating material.
So when you breathe helium, you are changing the harmonic structure of the vibrating "air". Since the speed of sound in helium is higher, the harmonic frequencies (called formants when discussing speech) shift to a higher frequency. Hence the squeaky voice.
Have you ever wondered if the opposite effect is possible? Can one breathe a gas that would make one sound a bit more like James Earl Jones?
I first saw this in 1995, and I've wanted to do it ever since. I need sulfur hexaflouride. The chemistry of SF6 is pretty awesome. Flourine is the most reactive of the halogens; I believe chemists say that it highly electronegative. Hydroflouric acid is so corrosive it can't be kept in glass or metal bottles. However, because it is so reactive, it can form extremely strong bonds. One substance using this to good effect is polytetrafluoroethylene, known commercially as Teflon. Teflon bottles are used to store hydrofluoric acid.
So SF6 forms such a tightly bonded molecule, that it is essentially chemically inert and not hazardous.
What happens when you breathe it? Watch Adam Savage from Mythbusters.
Freakin' cool , huh?
WARNING! Use extreme caution when attempting either helium or SF6. Both gases are suffocation hazards. Take small breaths from a balloon, and take several deep breaths afterwards. It's also a good idea to sit down, because you may become lightheaded. Never, ever attempt to breath directly from a pressurized tank. This is extremely dangerous. Bubbles of gas can form in your bloodstream which can block blood flow to the brain and cause a stroke. An Oregon teenager recently died from inhaling helium.