Why can't boiling water be boiled any more?

Boiled water can no longer be boiled, because it usually contains trace amounts of nitrate and heavy metal ions, such as lead and cadmium. When water is boiled repeatedly, the concentration of nitrate and heavy metal ions in water increases relatively due to the continuous evaporation of water.

Boiled water containing more nitrate will be further reduced to nitrite after entering the human stomach, and nitrite will destroy the function of blood to transport oxygen, making people's heart beat too fast, breathing difficult, and even fatal in severe cases. Similarly, heavy metal ions are harmful to human body.

So it's best not to quote boiled water repeatedly ~ ~ ~