The gas known as "the breath of life" is oxygen. Essential for the survival of most living organisms on Earth, oxygen is a colorless, odorless gas that we breathe in through our lungs. It plays a critical role in cellular respiration, a process by which cells convert glucose and oxygen into energy, releasing carbon dioxide as a byproduct. Most terrestrial life forms depend on oxygen, while aquatic ecosystems rely on oxygen dissolved in water. The Earth's atmosphere contains about 21% oxygen, which is produced by photosynthetic organisms, highlighting the interconnectedness of life and the environment on our planet.