To demonstrate the rise in frequency of vocal formants due to the increase in the speed of sound in a light gas.
Tank of helium with regulator and pressure gauges.
Take a couple of big breaths of air first to get plenty of oxygen, then breath out completely before inhaling the helium.
Snort a lungfull of helium and talk and sing normally. Your voice takes on a squeaky "Donald Duck" character due to the increase in frequency of your vocal formants.