Here's why:

* Natural Frequency: Every object has a natural frequency at which it vibrates most easily. This is determined by its size, shape, and material.

* Resonance: When an external force (like a singer's voice) vibrates at the same frequency as an object's natural frequency, the object absorbs energy from the force. This causes the object to vibrate with larger and larger amplitude.

* Shattering Glass: In the case of the glass, if the singer produces a note with the same frequency as the glass's natural frequency, the glass will start to vibrate intensely. If the singer sustains the note long enough and the amplitude of the vibrations is high enough, the glass could eventually shatter due to the stress caused by these vibrations.

In simpler terms: The singer's voice acts like a push on a swing. If the pushes are timed correctly (at the swing's natural frequency), the swing goes higher and higher. The same principle applies to the glass, and the "swinging" gets so intense it breaks.