You see, the sound created is not as terrible as the top opera singers, you can still . shatter a glass of water.
A trick that makes it easier to do this is to put a straw inside the glass and you can rely on the vibration level of the straw to adjust your voice. Wet your fingers and rotate evenly around the body of the cup to help you hear the continuous tone of the resonant capacity. At that time, the sound is usually up to octaves (the middle interval between a high sound and another high sound has a frequency equal to half or double) or more to break a glass.
However, if it's a legless cup, you won't be able to do that.
The reason is because the drinking glasses are quite special: narrow above, bulging in the middle. Therefore, you can hold the glass without affecting the resonant vibration of the cup.
In addition, the more expensive cups are brittle, because they are often made from crystal - extremely fragile materials, designed to be thinner than normal cups. The glasses of old wine are also more fragile, because they often have microscopic fractures in the molecule due to the effects of time.