The one kind of interaction between living things that is based on food is called a food chain. A food chain shows how energy passes from one organism to another in an ecosystem. It starts with a producer, such as a plant, which uses energy from the sun to make food. Then, a primary consumer, such as a herbivore, eats the producer. A secondary consumer, such as a carnivore, then eats the primary consumer. This process can continue with multiple levels of consumers, depending on the ecosystem.