Here's why:

* Photosynthesis is the process by which plants and some other organisms use sunlight, water, and carbon dioxide to create their own food (glucose) and release oxygen as a byproduct.

* Animal cells lack chloroplasts, the organelles that contain chlorophyll, the pigment necessary to absorb sunlight for photosynthesis.

* Animals are heterotrophs, meaning they obtain their energy by consuming other organisms.

Let me know if you'd like to learn more about photosynthesis or the differences between plant and animal cells!