Food chains are diagrams that show how energy is transferred when one living thing eats another living thing. Food chains always start with green plants. These plants use the sun’s energy to produce food by photosynthesis. They are called producers. In a food chain, anything that eats a producer or another animal is called a consumer. Animals that eat producers only are called herbivores. Animals that eat other animals only are called carnivores. Animals that eat either producers or other animals are called omnivores.