Why does helium make your voice squeaky?

The answer

Helium is about six times less dense than air, and sound travels nearly three times faster through it (about 927 m/s versus 343 m/s in air). When you inhale helium, the resonant frequencies of your vocal tract shift upward because sound waves bounce around faster in the lighter gas, amplifying higher harmonics. Your vocal cords actually vibrate at the same pitch, but the altered resonance of your throat and mouth filters the sound differently, producing that characteristic chipmunk effect.