Here's why:

* Photosynthesis is the process by which plants use sunlight, water, and carbon dioxide to create their own food (glucose) and oxygen.

* Carbon dioxide provides the carbon atoms needed to build glucose.

* During photosynthesis, plants absorb CO2 from the atmosphere through tiny openings in their leaves called stomata.

* The carbon atoms from CO2 are then combined with water molecules to form glucose, a sugar that provides energy for the plant.

So, in short, carbon dioxide is the key ingredient that plants "consume" during photosynthesis.