Here's the breakdown:

* Photosynthesis: This process uses sunlight to convert carbon dioxide and water into glucose (sugar) and oxygen.

* Water Splitting: During the light-dependent reactions of photosynthesis, light energy is used to split water molecules.

* Oxygen Release: The splitting of water releases electrons, hydrogen ions (H+), and oxygen gas (O₂). The oxygen is then released into the atmosphere as a byproduct.

So, the oxygen we breathe is ultimately a result of water being split during photosynthesis.