Here's why:

* Photosynthesis uses carbon dioxide (CO2) and water (H2O) as reactants, along with sunlight, to produce glucose (a sugar) and oxygen.

* The oxygen is a byproduct of the process and is released into the atmosphere.

Think of it like this: plants "breathe in" carbon dioxide and "breathe out" oxygen, just like humans breathe in oxygen and breathe out carbon dioxide.