In my opinion, the highlight of national day celebrations was shijie shouting "Ni nia eh!" after breathing helium.

Hence, I shall talk a bit about helium speech.

The fact that helium can change the frequency of our voice seem to show that our vocal cords are not really like string instruments, but more like woodwind instruments.

Our eardrums interpret the frequency, and not the wavelength of the sound as pitch. Thus, when a pure note is heard as higher pitch than another, it implies that the one of higher pitch also has higher frequency, but it says nothing of its wavelength.

The difference between string and woodwind instruments is that string instruments directly vary the frequency of vibrations in the air, whereas woodwind instruments directly vary the wavelengths of vibrations in the air. For example, the pitch of the sound from a violin string is determined by the frequency by which the string vibrates, whereas the pitch of the sound from a recorder is determined by the resonant frequencies of the air column in the recorder.

Since our interpretation of pitch is based on frequency and not wavelength, this means that the pitch of a string instrument is not affected by the speed of sound in the air, but the pitch of a woodwind instrument is. Since is the speed of sound is inversely proportionate to the square root of the density of the medium, the speed is sound is greater in helium than in air. Also, since, f = v/λ, the frequency of sound in a woodwind instrument is higher in helium than in air, since the wavelength is fixed.

Interestingly, deep sea divers cannot breathe air at high pressures, since at high pressures nitrogen induces an intoxicated state in the diver. Thus, they breathe a mixture of helium and oxygen, averting this problem somewhat. However, this also causes the voices of the divers to become higher. (Shijie should join the navy and become a deep sea diver someday, for the amusement of his colleagues.)

I would like to hear zilin or libing speak after breathing helium.

On a similar note, xenon is able to lower the pitch of peoples voice. It is more dangerous than helium, since it's high atomic mass makes its diffusion slow, allows high concentrations of Xenon to sit around and asphyxiate people. Although if you are really tall, you might more likely die of asphyxiation by helium than by xenon.

Just think about it. Hearing someone die of asphyxiation by breathing in helium must be pretty amusing. It would probably sound like happy tree friends. Or maybe a person dying from xenon asphyxiation would sound like Darth Vader at the end of Star wars III ("No...........").

In any case, if a potential Evil Overlord chances by this blog, I recommend you kill your adversaries with helium, and add some carbon dioxide to hear him/her struggle. Guaranteed laughter, which is good if you haven't quite mastered your evil laughter yet. Or you can use xenon instead, if you do get bored.