Here's why:

* Photosynthesis is the process plants use to convert light energy into chemical energy in the form of glucose.

* This process uses water (H2O) and carbon dioxide (CO2) as inputs.

* During photosynthesis, water molecules are split, releasing electrons, hydrogen ions (H+), and oxygen (O2) as a byproduct.

So, the oxygen we breathe is actually a waste product of photosynthesis!