Here's why:

* Photosynthesis uses carbon dioxide (CO2) and water (H2O) as inputs, and with the help of sunlight, converts them into glucose (a sugar) and oxygen.

* The oxygen produced is a byproduct of the light-dependent reactions in photosynthesis, where water molecules are split to provide electrons for the process.

While oxygen is the primary waste product, there are some other, less significant byproducts:

* Small amounts of heat are produced as a result of the energy transformations involved in photosynthesis.

* Some organic compounds may also be released, but these are usually recycled within the plant or used for other metabolic processes.

It's important to remember that oxygen is not a waste product in the sense that it's useless. It's crucial for respiration in most living organisms, including the plant itself!