Here's why:

* Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy (glucose) and release oxygen as a byproduct. This process requires sunlight, carbon dioxide, and water.

* Cellular respiration is the process by which all living organisms break down glucose to release energy in the form of ATP. This process requires oxygen and produces carbon dioxide and water.

Animals cannot perform photosynthesis because they lack the necessary structures (chloroplasts) and pigments (chlorophyll) to capture light energy. They rely on consuming other organisms (plants or animals) to obtain their energy.

Animals do perform cellular respiration, just like plants and other organisms. They use the glucose obtained from their food to fuel their metabolic processes.

Therefore, animals are heterotrophs, meaning they obtain energy from other organisms, while plants are autotrophs, meaning they produce their own food through photosynthesis.