[gus_massa]

I watched the video a few years ago.

Before watching the video, I though the trick to break the cup was to sing very loud and very close to the tuned to natural resonance frequency of the glass, so the movement of the glass and the air inside are so big that breaks the resistance of the glass suddenly. Like swinging a swing until you got a 360°.

In the video I learned that the important part are the micro cracks of the glass. If you sign loud and tuned up, the micro cracks begin to grow, and grow, and grow, until they are so big that one of them is big enough to break the glass.

The differences are that it's also important to sing for a long time to make the micro cracks grow, and that the exact moment is difficult to predict because it's not when the singer is extremely tuned but when the invisible crack are big enough.

(I also learned that it's possible to cheat and make a small crack on the border to break the glass faster and consistently and impress your friends. (I broke a glass singing, so I never had an opportunity to used that trick anyway.))

[gus_massa]

Too late to edit:

> I broke a glass singing, ...

I * never * broke a glass singing, ...