Here's why:

* Reduction: In chemistry, reduction means gaining electrons.

* Carbon dioxide (CO2): In CO2, carbon is in a relatively oxidized state.

* Glucose (C6H12O6): In glucose, the product of photosynthesis, carbon has gained electrons and is in a reduced state.

During photosynthesis, the energy from sunlight is used to split water molecules, releasing electrons. These electrons are then used to reduce carbon dioxide into glucose.