Why are ceramic cups used to knocking holes with sharp tools when they are filled with water upside down?

When the ceramic cup is filled with water, it is customary to use sharp tools to knock holes, because the cup is under unbearable pressure.

In our daily life, I believe many friends have used ceramic cups when making tea. Whether the ceramic cups we use are harmful, of course, not all ceramic cups will be harmful to human body.

Precautions for ceramic cups:

Ceramic cups are harmful, not toxic, but refer to lead and chromium dissolved in the cups during use. Lead and chromium are heavy metal elements. If these elements enter the body with diet, it will do great harm to the body.

Because this heavy metal element can't be eliminated by the human body, it will accumulate in the body and damage the normal function of the human body, so ceramic cups are still used less.