A food chain is the sequence of who eats whom in a biological community to obtain nutrition.

1. A food chain starts with the primary energy source, usually the sun or deep sea vents.
2. The next link in the chain is an organism that make its own food from the primary energy source -- an example is plants that make their own food from sunlight (using a process called photosynthesis). These organisms are called autotrophs or primary producers.
3. Next come organisms that eat autotrophs; these organisms are called herbivores or primary consumers -- an example is a rabbit that eats grass.
4. The next link in the chain is animals that eat herbivores; these are called secondary consumers -- an example is a snake that eats rabbits.
5. In turn, these animals are eaten by tertiary consumers -- an example is an owl that eats snakes.
6. The tertiary consumers are are eaten by quaternary consumers -- an example is a hawk that eats owls.
7. Each food chain end with a top predator, an animal with no natural enemies (some examples are alligators, hawks, and polar bears).
8. When any organism dies, it is eventually eaten by detrivores (like vultures, worms and crabs) and broken down by decomposers (mostly bacteria and fungi), and the exchange of energy continues.

The arrows in a food chain show the flow of energy, usually from the sun to a top predator. As the energy flows from organism to organism, energy is lost at each step.