* Lack of oxygen: The air we breathe contains about 21% oxygen, while the water we live in contains only about 0.003%. This means that there is not enough oxygen in water for us to breathe.

* Water pressure: The pressure of water increases as you go deeper, and this can make it difficult for us to breathe. At a depth of just 33 feet (10 meters), the pressure is twice what it is at the surface, and this can cause our lungs to collapse.

* Lungs are not designed for water: Our lungs are designed to breathe air, not water. When we try to breathe water, the water fills our lungs and prevents us from getting oxygen.

There are a few ways that humans can overcome these challenges and breathe underwater, such as using scuba diving gear or a snorkel. However, these methods are not without their risks, and it is important to be properly trained before attempting to breathe underwater.