We need a variety of materials to produce energy, but two of them are absolutely essential: glucose and oxygen. Glucose is produced by the body from food. Oxygen, on the other hand, cannot be produced by the body, so it must be taken into the body from the air through breathing.
The body can store some glucose. Therefore, even if you do not eat for several days, you will not die. Oxygen, however, cannot be stored. Therefore, if we do not always breathe and take oxygen into our bodies, we will die soon.
When you dive into the water, you can hold your breath for a little while. This is because we can take in oxygen from the air in our lungs for a short time.

Oxygen has the power to kill cells, and even today, bacteria (anaerobic organisms) and other organisms that die when exposed to oxygen exist underground and in the deep sea. Organisms that do not mind being exposed to oxygen (aerobic organisms) have mechanisms in their bodies to protect themselves from this harmful oxygen. One such mechanism is the enzyme superoxide dismutase (SOD), which converts oxygen into harmless substances.