Here's why:

* Photosynthesis is the process where plants use sunlight, water, and carbon dioxide to produce glucose (sugar) for energy and oxygen as a byproduct.

* The chemical equation for photosynthesis is:

* 6CO2 + 6H2O + sunlight → C6H12O6 + 6O2

* This means six molecules of carbon dioxide and six molecules of water react with sunlight to produce one molecule of glucose and six molecules of oxygen.

While plants use some of the oxygen they produce for their own respiration, they release the excess into the atmosphere, making it available for other organisms to breathe.