This party stunt almost never fails to amuse when it gives
a chipmunk-like voice to even the most masculine man. I've
often wondered who originated this stunt, and what else did
they suck up in their quest for the funny voice?

The secret to this trick lies in the fact that helium is
less dense than air. The less dense the medium, the faster
your vocal chords will vibrate, in this case making your
voice sound higher pitched. A lower pitch can be achieved
by inhaling a gas that's heavier than air, such as xenon,
but a party decorated with xenon filled balloons doesn't
sound as fun.